Drag in cycling is the combination of the resistive forces generated by pushing you and your bike through the air.
The forces will be from:
High pressure areas where the air first makes contact with you and the bike. You are trying to create a hole in the air to pass through. The air has to go round you rather than through you.
A friction effect as the air interacts with your skin, the texture of the fabrics you are wearing, and interacts with the bike frame, your wheels and other components.
A low pressure area behind you and the bike (mostly behind your rear end above the saddle) that will also exert a force.
The unit of force is Newtons but you might also see the drag expressed in grams.