Convergence is the behavior of both eyes to angle towards an object you are looking at. As the object gets closer to your face, your eyes become angled towards each other, and as the object moves away they become more parallel. This is an important part of depth perception and convergence problems can be a source of double vision.
A common childhood convergence problem is lazy eye. See a doctor for any convergence problems, find one that does vision training if you can. There are also surgeries to correct some convergence problems.