As an example, you can have one RGB contract that defines and works with only one asset. With time, this asset will create some history. You can also have multiple contracts, each of which will own multiple instances of some asset under it, and will be working together, creating their histories etc. All these contracts, histories and related data create what we call Stash. Basically, a stash is all the data related to the client-validation of RGB data that is owned and held by a specific party (e.g. a wallet).