Conditional and unconditional, in plain terms

An unconditional offer means the institution already has everything it needs, and the place is yours if you accept. A conditional offer means the place is held for you provided you achieve or produce specific things by a specific date — usually final grades, sometimes a language result, sometimes a document that has not arrived yet.

What people miss is that a conditional offer is a real offer, not a maybe. It is a commitment with a list attached. Read the list on the day it arrives, decide honestly whether each item is within reach, and if one is not, say so early. Institutions can occasionally adjust a condition months out. They almost never can in the final fortnight.