As an adjective, fast means swift or quick.
- She is a fast runner.
- The internet is very fast.
But fast also means firm or without movement.
- The farmer made fast the door to the storage shed.
- He is my fast friend and has stood by me in difficult times.
- The desk drawers were stuck fast.
The second meaning according to Merriam-Websters Online dictionary is the original meaning historically.
When we use fast as a verb, it means to eat little or no food.
Christians fast during lent, and Moslems fast during Ramadan.