Equity, also referred to as expected value (EV), is your share of the prize pool at any point in a tournament. For a given hand, the SitNGo Wizard compares the equity you would have if you fold with the equity you would expect to have if you raised all-in. The difference between these two values tells you which is the better play.

This article has a nice discussion of expected value.

What is Expected Value (EV)? by Scuba Chuck