Tennis elbow is inflammation or microtearing of the tendons that join the forearm muscles on the outside of the elbow.

It is overuse injury of forearm tendon a/k/a Common Extensor Tendon due to repeated chronic overuse.
Most common symptoms is pain on outside part of elbow which increase during gripping activities.
Unable to hold objects/ drive etc.
Most commonly associated with sports such as tennis/ badminton/volleyball etc.

Tennis elbow is often due to damage to a specific forearm muscle.
The extensor carpi radialis brevis (ECRB) muscle helps stabilize the wrist when the elbow is in Extension.

This occurs during a tennis groundstroke, for example. When the ECRB is weakened from overuse, microscopic tears form in the tendon where it attaches to the lateral epicondyle. This leads to inflammation and pain.