A c-bet (continuation bet) is a flop bet by the player who made the last raise preflop: it continues the aggression, whether they hit or not.
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 C-bet
Continuation betting too little gives free folds to hands that missed. Betting almost every time is a leak too: on boards that favor your opponent (low and connected when you opened from UTG, for example) you should check part of your range.
Compare it with your opponents' fold to c-bet: against players who fold a lot, bet more.
If your C-bet is out of range
Too low
You rarely continuation bet: you give free folds to hands that missed the flop.
Too high
You c-bet almost every time. On boards that don't favor you, it's better to check part of your range.
How many hands you need to trust it
Flopmortem doesn't flag your 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.