It's really easy
say you flop a medium strength hand like TPNK

you decide to cbet and your opponent calls
your opponent checks the turn and you check behind
then your opponent leads the river and you call

that's what's called pot control
if he has a better kicker, you put in two bets with top pair no kicker
if he has some draw that didn't get there on the river you caught his bluff

you lose the least when behind and still make money when you're ahead

if you did that your river aggression would be like 2
but since you always bet/fold like every street you end up putting three bets in with marginal hands that will only get action from better

note that you have to be in position to play these marginal hands since you can check one street behind... either the turn or the river usually
if you're OOP, you check to your opponent and might induce him to bet at you swelling the pot with a marginal hand... when you don't know if he has anything

this is why you can raise a hand like K8s from the button because you will probably be able to pot control if you flop a king since you're dumping the hand to any check/raise from your NL10/25 opponents