You raised preflop. They called. The flop comes. A continuation bet is you betting again — keeping the initiative. It is not “always bet every flop.”
The preflop raiser is supposed to have the stronger range. A c-bet wins a lot of pots when the flop misses everyone, and it builds the pot when you hit. A healthy c-bet frequency is about 55–65% — not 100%.
Boards that smash the caller (low connected, monotone) are bad auto-c-bets. Multiway pots shrink your c-bet range. Out of position versus a sticky player, checking back more medium hands is correct.
A double barrel is betting the turn after c-betting the flop. Do it when the turn is bad for their calling range (overcards, flush-completing if you have the nuts or a blocker bluff) or when your draw still has equity.
Fold to c-bet over ~55% and they print by firing any two. Too low and you float trash. Defend with pairs, strong draws, and some floats that have a plan for the turn.