If the data in your database is sensitive, you need to control who may have access to it. If the database is to be used unsupervised, you may wish to control which parts of the database the user can have access to.
Depending on the way the database is to be used, you have a range of options for protecting it, when using Access. You might set a password for entering the database, or set different passwords for different users. You might create forms which limit the ways that a user can work on the data.
A range of techniques is described in this article, and illustrated in a sample Access database.