A steal is an open from late position (CO, button or SB) when nobody has entered before. Fold to steal in the BB is the percentage of times you fold your big blind to those steals.
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 steal in the BB
The big blind is the position that loses the most in the long run: the key is to lose as little as possible. Since you've already put in 1 bb, you're getting a good price, so you can defend lots of suited and connected hands.
If you fold too much, you hand over your blind every time someone opens from the button. If you defend too much, you play lots of pots out of position with hands that don't make it to the river.
If your Fold to steal in the BB is out of range
Too low
You defend too much in the BB against steals: it's the worst position postflop.
Too high
You fold too much in the BB to steals. You're getting a good price: defend more suited and connected hands.
How many hands you need to trust it
Flopmortem doesn't flag your Fold to steal in the BB 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.