In a top-loading washer, the agitator is the arm in the center of the machine that helps move your clothes around while washing. By agitating your clothes in the soapy water, this component removes dirt from your clothing. In addition to shaking your clothes through the water, the agitator makes the clothes rub against each other which also forces out dirt.
In a front loading washer, there’s no agitator. Instead, the drum rotates back and forth to agitate the clothes. Some newer top-loading washing machines may function without a traditional agitator and instead use a wash plate to help toss clothes around.