According to Wikipedia:

The name of the strategy refers to the way ink spots spread on a piece of blotting paper or tissue, starting as tiny scattered points but spreading to cover most or all of the paper.

A fine article by Farnam Street gives a good example (excerpt) of it’s implementation by Wal-Mart from the book ‘Sam Walton: Made In America’

Now that we were out of debt, we could really do something with our key strategy, which was simply to put good-sized discount stores into little one- horse towns which everybody else was ignoring. In those days, Kmart wasn’t going to towns below 50,000, and even Gibson’s wouldn’t go to towns much smaller than 10,000 or 12,000. We knew our formula was working even in towns smaller than 5,000 people, and there were plenty of those towns out there for us to expand into. When people want to simplify the Wal-Mart story, that’s usually how they sum up the secret of our success: “Oh, they went into small towns when nobody else would.” And a long time ago, when we were first being noticed, a lot of folks in the industry wrote us off as a bunch of country hicks who had stumbled onto this idea by a big accident.

Maybe it was an accident, but that strategy wouldn’t have worked at all if we hadn’t come up with a method for implementing it. That method was to saturate a market area by spreading out, then filling in. In the early growth years of discounting, a lot of national companies with distribution systems already in place — Kmart, for example — were growing by sticking stores all over the country. Obviously, we couldn’t support anything like that.

But while the big guys were leapfrogging from large city to large city, they became so spread out and so involved in real estate and zoning laws and city politics that they left huge pockets of business out there for us. Our growth strategy was born out of necessity, but at least we recognized it as a strategy pretty early on. We figured we had to build our stores so that our distribution centers, or warehouses, could take care of them, but also so those stores could be controlled. We wanted them within reach of our district managers, and of ourselves here in Bentonville, so we could get out there and look after them. Each store had to be within a day’s drive of a distribution center.

We saturated northwest Arkansas. We saturated Oklahoma. We saturated Missouri. We went from Neosho to Joplin, to Monett and Aurora, to Nevada and Belton, to Harrisonville, and then on to Fort Scott and Olathe in Kansas — and so on. Sometimes we would jump over an area, like when we opened store number 23 in Ruston, Louisiana, and we didn’t have a thing in south Arkansas, which is between us and Ruston. So then we started back - filling south Arkansas. In those days we didn’t really plan for the future. We just felt like we could keep rolling these stores out this way, and they would keep working, in Tennessee, or Kansas, or Nebraska — wherever we decided to go. But we did try to think ahead some when it came to the cities. We never planned on actually going into the cities. What we did instead was build our stores in a ring around a city — pretty far out — and wait for the growth to come to us. That strategy worked practically everywhere. We started early with Tulsa, putting stores in Broken Arrow and Sand Springs. Around Kansas City, we built in Warrensburg, Belton, and Grandview on the Missouri side of town and in Bonner Springs and Leavenworth across the river in Kansas. We did the same thing in Dallas.

This saturation strategy had all sorts of benefits beyond control and distribution. From the very beginning, we never believed in spending much money on advertising, and saturation helped us to save a fortune in that department. When you move like we did from town to town in these mostly rural areas, word of mouth gets your message out to customers pretty quickly without much advertising. When we had seventy-five stores in Arkansas, seventy-five in Missouri, eighty in Oklahoma, whatever, people knew who we were, and everybody except the merchants who weren’t discounting looked forward to our coming to their town. By doing it this way, we usually could get by with distributing just one advertising circular a month instead of running a whole lot of newspaper advertising. We’ve never been big advertisers, and, relative to our size today, we still aren’t. Just like today, we became our own competitors. In the Springfield, Missouri, area, for example, we had forty stores within 100 miles. When Kmart finally came in there with three stores, they had a rough time going up against our kind of strength.

So for the most part, we just started repeating what worked, stamping out stores cookie-cutter style. The only decision we had to make was what size format to put in what market. We had five different store sizes—running from about 30,000 to 60,000 square feet — and we would hardly ever pass up any market because it was too small. I had traveled so much myself looking at competitors in the variety store business that I had a good feel for the kind of potential in these communities. Bud and I knew what we wanted in the way of locations. Like so many of the ideas that have made our company work from the beginning, we’re still more or less following this same strategy, although today we’ve moved into some cities outright. But I think our main real estate effort should be directed at getting out in front of expansion and letting the population build out to us.

Emphasis, mine.

Seems like a brilliant read! Doesn’t it? Well you can buy it.