I would even expand Cathy's definition somewhat to include watering widely as well as deeply. Here's why:
If I understand it, any tree, but especially a recent planting, needs a root system that is capable of replacing all the water being transpired by its leaves. It's like cash flow--basically a balancing act between supply and demand. So, for any fig newly planted in the ground, the root system won't extend any farther than it did in the pot, simply because the roots haven't had enough time to grow--but they'll grow very quickly if conditions are right. However, if you only give it limited water, like I did, it won't wet the soil very far beyond the original root ball, so even if the roots themselves are moist, the plant is still essentially "in a pot" because the soil surrounding the rootball is too dry for the roots to penetrate those areas. When it's hot, dry or windy, the tree won't be able to take up water fast enough to replace what it's losing through the leaves, and it will get badly stressed.
My drip emitters were close to the trunk, and probably did an adequate job of soaking the soil directly below the tree, but the soil off to the sides still stayed pretty dry. This limited the volume of moist soil available for new roots to grow into--basically they could only grow straight down. But, if the soil out away from the tree is also kept moist, the roots will keep growing horizontally as well, and will soon extend a long way from the trunk. Making a shallow, broad soil basin around the trunk and filling it to the brim with water will ensure a broad root system; keeping it full long enough to soak in deeply as well will ensure a deep root system. You want both.
Before rushing out to soak your new plantings, however, it's important to remember that everybody's soil is different. I've got bone-dry, sandy, fast-draining soil. It's literally impossible for me to overwater, but I see posts from other parts of the country where figs are drowning due to constant rain and clay soil, and people are trying to figure out how to get rid of the excess moisture. What works well in one location may be the worst thing you could possibly do in yours!