officially - nothing. it is automatically multi-user. (Unlike excel where you get "another user opened this ... do you want this to be read-only")
however, unofficially - the problem is that is generally ill-advised for multiple users to open the same physical copy of a database.
therefore the perceived wisdom is to SPLIT the database. put the tables in a BACKEND database, and the forms and code in a FRONTEND database, and link the frontend to the backend
Now each user needs a separate copy of the FRONTEND.
The other advantage of splitting databases is that you can amend the design of your forms etc, independently of other users - which is hard to do in an unsplit database.