What it solves
One player moves all in first, or folds; everyone left to act calls or folds. The solver finds the ranges where no player can gain by changing their own: shove any wider and the callers punish it, call any wider and the shover gains. That point is the Nash equilibrium. Each hand's value comes from its preflop equity against the ranges that call, so no cards past the all-in matter.
A worked example
Heads-up with 10 big blinds each and no ante, the small blind shoves about 58% of hands and the big blind calls with about 37%. Make the stacks uneven and the chart changes: a shover facing two players with 3 big blinds each can shove much wider than one facing two 10 big blind stacks, because a short caller can win less. The link under the solver keeps your spot, so you can share it.
What a push/fold solver does not do
It does not play flops. Deeper stacks need raises smaller than all in, and postflop play, which a preflop all-in solver leaves out. For fixed stack depths, the push/fold charts are ready to study, and for deal-making the ICM calculator gives each stack's value. Solved flop spots are in the flop solver, and any other flop in the solved heads-up spots goes to Turnstone, the app's engine, on the analysis board. Both are invite-only for now.