A multi-instance deployment runs several copies of an app or service. Requests can be shared across the copies, so one busy or unavailable copy may not stop the whole service.
Imagine a shop opening several counters so more customers can be served. A system may use a traffic manager to direct each request to a working copy.
Running several copies adds complexity. The app needs a way to keep important information consistent, and the team needs to know what happens when a copy is added or removed.