This part is the replacement grommet for your cooktop or range. It is made of black plastic and is approximately 1 ¼ inches in diameter. This part may also be known as the cooktop burner knob gasket, ...
I had used a pan too large for the middle burner - result: melted knob. Just pulled the old one off (up and straight) and then pushed the new one down firmly. Make sure the indicator line is pointing at you, so the flat side of the spindle matched the flat side of the hole on the knob. Easy peasy.