A control arm is a rod kind of object that has a pivot at both ends. A car has quite a few of them, including the upper control arm and the lower control arms, which are organized to form the letter A. Furthermore. the arm upholds some important joints in position tightly.
If you ever get a feel of steering wheel vibration then it is one of the first symptoms commonly associated with the bad control arms. If the bushings or ball joints in it turn out to be excessively worn it can lead to wheel shimmy, which may cause extreme vibrations that can be felt strongly in the Steering wheel.
It is also well-known as an A-arm, which is a hinged suspension link in the middle of the chassis and the suspension upright or hub that conveys the wheel. These are part of the suspension system too.