Solving the first layer (abbrevaited FL) is an important part of LBL methods for any size cube. It can be solved using intuition only, and it gives a satisfying sense of accomplishment for a beginning cuber, so for that reason it is popular for teaching people their first method to solve the 3x3x3. The first layer is also an important part of the Waterman method.
For the 3x3x3, there are several ways to solve the first layer. The most basic way, and the typical way for a pure LBL method, is to first create a cross and then fill in the corners one by one. It is also possible to create the first layer using block building, similar to the first block in the Roux method. The Waterman method takes yet another approach, solving the four corners first and then filling in the edges and center at the same time.