A project groups everything for one stream of work. You add the files it leans on, write project instructions as a standing brief, and keep all its chats in one place. Every new chat inside starts with that context.
Project instructions take priority over your custom instructions inside the project. Each project has a memory setting: default, or project-only to fence it off. On Business you can share a project with colleagues, with Edit or Chat access. Shared projects always use project-only memory.