An adjustable strike plate is one that lets you move the position of the opening the latch or the bolt goes into, usually with a screw or an eccentric cam, without having to take it off the frame. That way you can set how firmly the door closes against its seal.
It is very common on aluminium and uPVC doors, and it is also fitted to timber doors that react to damp and move slightly over the course of the year.
If you notice the door closing very loosely and rattling, or it is hard to shut and the key feels stiff, sometimes adjusting the strike plate is all it takes. Do it carefully: if it is set too tight, the lock works under strain and suffers. More under lock installation.