'F2L' refers to the 'First Two Layers' of the 3x3 cube, or essentially all layers up until the last layer on larger cubes.
There are many ways to solve the 'F2L' on a cube. A common system is using the Fridrich method first two layer approach. After solving the cross, a corner-edge pair is paired up, and then inserted into the correct slot. A total of four corner edge (or 'CE') pairs are made and inserted to solve the first two layers.
Another way to solve the 'F2L' is by building blocks, common during the first two layers of the Petrus method.