Value minus every mortgage, HELOC, lien and back tax against it, then minus what it costs to sell, so the number left is what a sale would put in your hand.
Cost to sell is an editable assumption covering commission, closing costs and any repairs a buyer would ask for. Change it if your situation looks different. If total debt exceeds the home's value, gross equity and the reachable figure both show as negative, which means a sale at this value would not cover what is owed.
Gross equity is what the house is worth minus what is owed against it. Nobody collects that number directly. Selling costs money, in commission if you list it or in the margin a cash buyer needs, so the figure that actually reaches you is smaller.
That is a real position, not a calculator error: a sale at today's value would not clear what is owed. A short sale, a delay, or a lender conversation are the usual next steps, and none of them are shameful.