Lock file (Microsoft Access)
LDB is a file extension for a lock file format used by Microsoft Access database to indicate records in a shared database that are in use by another user. The LDB file also stores information about what user has the record in question locked.
The purpose of a record lock is preventing users from editing the same data simultaneously and thereby avoiding unexpected data loss or corruption.
Learn more about LDB files:
SearchSQLServer.com offers this Fast Guide to avoiding data integrity gotchas.
IT Knowledge Exchange maintains a FAQ about record locks.