This relief is a piece from the upper inner courtyard of the Pergamon Altar, a famous Hellenistic marble relief series that tell the life story of the mythical hero Telephus.
In Greek mythology, Telephus was the son of Heracles (in some countries, Hercules) and the priestess Auge.
Abandoned on a mountain by his fearful grandfather King Aleus, but saved and raised by a deer.
When he grew up, he returned to Mysia (a small part of the current Asia Minor), and later became the king.
The title of this specific section (Panels 11 and 12) is the Arming of Telephus.
His right hand is squeezing Auge's clothing to signify a plea for recognition or a desperate bond, which is a key narrative turning point in the myth.
At this moment in the story, Telephus and Auge do not yet realize they are mother and son.
Auge, having been banished to Mysia, was ordered by her adoptive father to kill the stranger (Telephus) who had arrived in the kingdom.