When you are defensively betting your goal is to have your opponent call or fold only so they dont take the lead or rase your hand.
Bet defensively when you are on a draw and have not made your hand yet.
I.e. we played 7,8s in middle position. On flop we get a flush draw. Checking in that situation is ok but it might not be the best decision because we give up potential value if we hit that draw.
But this would give our opponent the chance to make a bet which would force us to fold.
The trick to betting defensively is to make a bet big enough for them to call or fold, but cheap enough that it doesnt cost you too much to see the next card.
Usually a bet around 1/3 the size of the pot is the right bet.