One square of the chart: the best action for a hand against an upcard.
The symbols are S (stand), H (hit), D (double; hit if you can't double; Ds means stand if you can't double), P (split), and U (Double Up; Ud means double if you can't DU). Example: the square where the hard 16 row meets the dealer 10 column is H. Squares where the EV gap is small are called "boundary" cells. Tap a square to see the EV of each action.