Toggling "Drag View" allows a builder to view the air resistance that their aircraft generates. Generally, the darker red a part is displayed, the more drag it has. Drag is generated by flat, wide surfaces. Drag can be minimized by adding nosecones, which reduce the surface area on which drag can be generated.
Drag is presented in abstract "drag points".
Please note, more drag is actually a good thing on parts such as inlets, which require airflow to function properly.