Fold to 3-bet is the percentage of times you fold your hand after opening and facing a re-raise.
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 3-bet
If you almost always fold, any opponent can 3-bet you with any two cards and win. Defending means calling in position with hands that play well postflop and mixing in some bluff 4-bets (A5s, A4s).
If you defend too much, you end up playing big pots out of position with dominated hands: KJ or QT lose money against a strong 3-bet range. In full ring, 3-bets are stronger, so it's normal to fold somewhat more than at 6-max.
If your Fold to 3-bet is out of range
Too low
You defend too much against 3-bets: out of position, that loses money. Fold your dominated hands.
Too high
You fold too much to 3-bets: any opponent can 3-bet you with almost anything. Defend more (call in position, bluff 4-bet with A5s/A4s).
How many hands you need to trust it
Flopmortem doesn't flag your Fold to 3-bet as a leak until it has at least 30 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.