Microsoft Access runtime mode database file (.accdr) files are simply renamed (.accdb) Access database files. By changing the database's file extension from .accdb to .accdr, users can create a "locked-down" version of their Microsoft Access database. When the extension is changed back to .accdb the database has restored its full functionality.
This is usually done in order to prevent accident changes to the database, and is more or less what accdr to accdb conversion represents.