Focus is something that camera lenses and sleepy students are always being asked to do. For cameras, it means finding a point where the subject is clear or "in focus." For students, it means paying attention.
Focus is all about finding a center — of a parabolic curve, of a lens, of a meditative state. In Latin, focus meant 'domestic hearth,' which just goes to show that not much has changed — since kitchens remain the focus of the modern home. Focus can be used as a verb, as in "I need to focus on my work, so I can play video games later;" and as a noun, as in "What is the focus of this essay? I can't tell, since the writer seems to be all over the place."