Fold to c-bet is the percentage of times you fold on the flop when the preflop raiser makes a continuation bet against you.
How it's calculated
The healthy ranges on the side are guidelines for NL Hold'em cash games with 100 bb stacks, the same ones Flopmortem compares your game against. Use them to spot big deviations, not as an exact target: they depend on the table, your opponents and your style.
How to read your Fold to c-bet
It's one of the most common leaks: if you fold too much, your opponent makes money betting any two cards. Defend more with pairs, draws and overcards with backdoors.
If you call almost everything, you'll reach lots of turns with weak hands and lose more on later streets; it shows up as a high WTSD and a low W$SD.
If your Fold to c-bet is out of range
Too low
You call too many c-bets: you'll reach lots of turns with weak hands.
Too high
You fold too much to c-bets, and opponents exploit it by betting any hand. Defend more with pairs, draws and overcards.
How many hands you need to trust it
Flopmortem doesn't flag your Fold to c-bet as a leak until it has at least 40 opportunities. From there it calculates it with its 95% confidence interval: if the whole interval falls outside the healthy range, the leak is confirmed; if only the measured value is outside, it's a trend. The guide how many hands you need to trust your stats explains it.