A depot is a place where things are stored in large quantities for future use. Your office might have a depot in the back where supplies are stored.
Depot came into English from the French word dépôt, meaning "a deposit, place of deposit." A depot can be a place like a warehouse where items are temporarily deposited, but we also use it for a train or bus station. In this case, think of it as a place where a many passengers are being temporarily housed. Your bus trip might require you to get off one bus at a big depot, and transfer to another.