There are many reasons to file a motion. Motions ask the court to enter an order letting you do something or requiring the other side to do something. 

You could file a motion to ask for the court's permission to do something. There are many motions that do this, such as a Motion for a Continuance. A Motion for a Continuance asks the court to postpone a court date because you will be unable to attend or because you will not have the information you need. If granted, the court will give you a new court date. Other examples are if you need more time to move in an eviction, or you need the court to change an order.

Another type of motion is a motion to ask the court to force the other side to do something. An example of this type of motion is a Motion to Compel Discovery. You would file this motion if you have asked the other side to turn over documents or answer questions during discovery and they refuse. This motion asks the court to force the other side to turn over those documents.

You cannot file a motion if you are trying to delay proceedings, cause trouble for the other side, or for other improper reasons. If you do, the court may sanction you. A sanction is a penalty - usually a fee or a fine.