Imposition means you force something — usually an unwanted burden — on someone else. If your friends invite themselves over for dinner the night before your big test, that's an imposition because you'll have to cook instead of studying.
Imposition comes from the 14th century Old French word of the same spelling, meaning "the levying of taxes, a tax, duty." Taxes are a good way to remember the meaning of imposition. You never want to pay taxes — but you have no choice but to pay. It can be the same for other kinds of burdens: if something is an imposition, you don't want to do it, but you likely will have to.