What is MDF and LDF files stand for in SQL server?

Each SQL database will have MDF and LDF file.

• MDF is a primary database file - which stores the data
• LDF is a log file which stores the transaction log of that database.

Both files are necessary to restore a database.

Reference:

http://sqlserverpedia.com/blog/sql-server-tutorial/what-is-meant-by-ldf-and-mdf/