A stockpile is a large amount of something that you save for later. If you're worried about running out of your favorite flavor of bubble gum, you might decide to keep a stockpile of it.
If you hear the word stockpile on the news, it's probably referring to weapons which are accumulated and saved by a country that might need to use them some day in the future. You can use stockpile to describe the actual cache of weapons and also as a verb, to describe the act of storing them. The word was originally a mining term that literally described a pile of ore. During World War II, stockpile became a more general term.