W$SD (Won $ at ShowDown) is the percentage of showdowns you win: of the times you get to the end and the cards are shown, how many pots you take.
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 W$SD
A low W$SD means you reach the end with hands that are too weak: you call big river bets hoping for a bluff that doesn't come. A very high one can mean you play too tight and fold hands that would have won.
It makes sense to look at it together with WTSD; with few hands, it's one of the stats that swings the most.
If your W$SD is out of range
Too low
You win little at showdown: you call with hands that are too weak.
Too high
You win a lot at showdown: you may be folding hands that would have won (playing too tight).
How many hands you need to trust it
Flopmortem doesn't flag your W$SD 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.