The lateral pterygoid is located deep inside your jaw, and is a key muscle in the function of chewing or mastication. It function is pull the condyle forward, in essence opening the jaw.
The muscle itself is located deep inside the face and it not easily accessible. The actual position is inside of the jawbone, making massage very difficult from the outside.
The lateral pterygoid can be reached inside of the mouth via this method.
Here's a great video from KenHub on how the muscle works. This muscle has a bad reputation for being a key culprit in those suffering from TMJ or TMD pain.