How often you re-raise a preflop opener — facing exactly one raise. A 4-bet is a different stat and is not counted here. ~7% is a typical solid 6-max baseline.
A 3-bet is the FIRST re-raise preflop (the open is the 1st raise, the 3-bet the 2nd). 4-bet spots are excluded. It builds pots with your strongest hands and applies pressure with select bluffs. Under ~4% and opponents can open into you risk-free; over ~11% and you are re-raising too light.