Your elbow joint is where three bones in your arm meet: your upper arm bone (humerus) and the two bones in your forearm (radius and ulna). It is a combination hinge and pivot joint. The hinge part of the joint lets the arm bend and straighten; the pivot part lets the lower arm twist and rotate.
At the upper end of the ulna is the olecranon, the bony point of the elbow that can easily be felt beneath the skin.