Someone fell on the outside of my ankle and i heard a pop. not that long after it was hard to rotate my ankle and it hurt on the outside. what is it?

Sprain?: Could be sprained ankle, or tendon injury, or even a type of simple fracture. Without and exam and maybe even some testing (X-ray) it is quite impossible to tell you over the computer. best for you to get the ankle evaluated. Good luck.

Tendon injury: Sometimes when you hear a pop around a joint it is from a tendon getting pulled across a bone. This can injure the tendon and cause swelling and pain on movement of the joint. Try ice, rest, compression (ace wrap) and elevation of the ankle. If it does not improve greatly in a week or so see your doctor. If you have trouble bearing weight on the foot when walking see a doctor immediately.