It is a rough estimate of the expected outcome of all your coins staking. Of course, it's not possible to stake all of them, as there is competition, but the higher the value is, the more probable your coins will stake soon and produce more stake reward.

It is calculated by multiplying the coin pile size and the age of each coin pile eligible to stake (more than 9 days old).

This is going away in Diamond v3 however, as coin age will not be considered anymore there.