Did you guess the name of this tool? It is the Ball Joint Press. Just as the name implies, the tool presses the ball joints from a control arm so they can be replaced.
Vehicles have upper and lower ball joints. Worn ball joints are a safety issue. The front suspension will come apart; causing you to lose control of your vehicle. Yikes! Don’t put off this repair.
The video below does an excellent job of showing you a ball joint and how the press works.