In gardening, people use the term "drip line" in two different ways. One refers to an imaginary line created by the circumference of a tree's longest branches. The other is a literal line, a type of hose laid down for watering. The type of drip line under discussion is usually clear from the context.
In the first sense, the drip line is an important thing to understand when establishing and watering trees. Imagining a tree with branches that grow in perfect symmetry, the drip line creates a circle at the widest point of the branches. The ground inside that circle is inside the drip zone.
Trees have evolved to utilize water efficiently and creatively. High concentrations of water show up right at the drip line because water shed down the leaves of the tree drips off the leaves at the ends of the biggest branches. The tree has a number of rootlets that extend to the drip line to take advantage of this supply of water. Consequently, trees prefer to receive water at the drip line, rather than at their roots. Watering at the roots can actually cause rot and disease.
In the sense of a line used for irrigation, a drip line is a hose marked with small holes. When water flows through the hose, it leaches out of the holes. This allows people to deliver water across a wide area over a long period of time, providing deep watering to trees and plants. Watering slowly gives the soil time to absorb the water, while trees and plants take it up through their roots, in contrast with dumping large amounts of water all at once. When too much water is applied, much of it is lost by evaporation before it gets to the plants.
People can set up automatic drip line irrigation for a low-maintenance garden. In this irrigation scheme, the hoses are left in place and operate on a timer or activate when the gardener turns them on. It's also possible to drag out drip lines for specific irrigation tasks. The lines tend to last longer this way because they are not exposed to the elements by being left out, but it can be a nuisance to untangle and lay out drip lines for every watering session.
Gardening stores usually supply a range of irrigation products including drip lines and timers. People can also make their own by carefully puncturing a hole in a hose. The size and intervals of the punctures can be adjusted for different irrigation needs.