On an office chair that has a rocking option where you can rock the office chair back and forth or recline the knob on the office chair is for tension of the backward or rocking motion of the office chair.
When you turn the knob counter clockwise it releases the tension so that the chair reclines easier and further back and allows you to rock the chair back and forth easier.
When you turn the tension knob on the office chair clockwise it increases the tension on the office chair so that there is more resistance when you rock back and forth.
So the chair doesn't rock back and forth as easy.
Some office chairs may have a lumbar support option as well but that knob is usually on the side of the office chair which pushes out against the back of the chair or returns to the inside of the back of the chair depending on where you like it set.