The verb conclude means two related things: coming to a decision, and bringing to a close.
Conclude is related to the word close, which is related to an old Latin word for "barrier" or "bolt" — the kind you put on a door. When you conclude something, it's as though you're pulling a door shut and bolting it. If your girlfriend concludes an argument by leaving the room and slamming the door, you can conclude that she's angry at you.