When you stash something, you stow it away to use it later. Your little brother might stash his Halloween candy under his bed, for example.
Use the word stash as a verb that means "hoard" or "stockpile," or as a noun — a pirate's secret pile of gold doubloons can be called a stash, and so can the collection of overdue library books you've got piled beside your bed. The verb version of stash was originally a late 1700's criminals' slang word meaning "conceal." It may have come from a combination of stow and cache.