Stolen from Access help:
If your database contains Microsoft Visual Basic for Applications (VBA) code, saving your Microsoft Access database as an MDE file compiles all modules, removes all editable source code, and compacts the destination database. Your Visual Basic code will continue to run, but it cannot be viewed or edited. Saving your database as an MDE file secures you forms and reports without requiring users to log on or requiring you to create and manage the user accounts and permissions that are needed for user-level security.
Your database will continue to function normally—-you can still update data and run reports. Additionally, the size of the database will be reduced due to the removal of the code, so memory usage is optimized, which will improve performance.
Saving your Access database as an MDE file prevents the following actions:
Viewing, modifying, or creating forms, reports, or modules in Design view.
Adding, deleting, or changing references to object libraries or databases.
Changing code — an MDE file contains no source code.
Importing or exporting forms, reports, or modules. However, tables, queries, data access pages, and macros can be imported from or exported to non-MDE databases.