When something "is accounted for", it means that you know where it is. You usually use this phrase to talk about a group of things, like all of the students in your class:
Everyone's accounted for.
This means that you know where everyone is. They might not all be with you, but you know that, for example, 28 of them are in your class, 1 is at home sick, and 1 is in the school counselor's office.
Either people or things can be accounted for:
Before you leave, please make sure that all your valuables are accounted for.
The opposite of "accounted for" is "unaccounted for":
Two soldier have been killed, and three more are unaccounted for.
The phrase "make sure ___ is accounted for" is common, so you might want to memorize both parts of it together.