Tarzan hasn't been able to escape from the natives who rescued him from the mire so they could prolong his agony. In this scene, Tarzan is tied over the mud with one arm tied to a tree on one side and the other arm tied to a tree on the opposite side. His ankles are tied together, so, once he starts sinking, he cannot pull his legs from the mud and he just keeps sinking deeper until he is almost up to his armpits.

He struggles and pulls with all his strength, but he cannot escape. It looks like he will still be there when the natives return.