A multi-point lock is one which has multiple points of locking along the door/window frame. Typically, a multi-point lock has 3 or 5 points of locking (but this is not always the case). Multi-point locks are one of the most secure types of locks but can be tricky to fit, so leave this to a professional if you can.
Multi-point locks are a modern mechanism which is why they're commonly used on composite doors, but they can absolutely be used on uPVC and timber doors as well.