Memory lets ChatGPT carry what it learns about you from one chat to the next. There are two parts: saved memories, specific facts it has stored, and chat history, details it can draw from past conversations.
That is handy, and it can also go stale or wrong. You can read the memory summary, see where a memory came from, and delete what's off under Settings, Personalization, Memory. Deleting a chat doesn't delete a memory saved from it. For a clean slate, use a Temporary Chat. Projects can have their own, fenced-off memory.