The word reservoir refers to a place where large amounts of water get stored. It can also be used to describe great amounts of other things, such as when you refer to a trivia expert as a reservoir of useless knowledge.
The word reservoir comes from the French réservoir, which further derives from the Old French reserver, meaning "to reserve." Knowing this, it makes sense that the meaning evolved into a place, either man-made or natural, where water is stored or "reserved," usually for distribution for drinking, irrigating, and generating power. Remembering the related word reserve might also help in both the spelling and pronunciation of the word, as reserve is spelled (and pronounced) with "er," the same as "reservoir."