visayanbraindoctor: This is a classic looking KID attack, but note that it was played 1936! The KID then was not that popular. I wonder if Keres kad anything to do with the KID's resurgence later on in the 1950s.
The young Keres was always looking to engineer an attach. After
15. Nh4 he embarks on a pawn sacrifice with
15... Rf6 which is just begging for Schmidt to play 16. g4 Nf8 and 17. g5
Black can't take the pawn hxg5 because Bxg5 pins his Rook, and he has to move
So it must have seemed weird to Schmidt who probably could not see Keres' point at this time, and so plays along.
18. gxh6 Bf6 which suddenly clears Black's second rank. Suddenly Keres' intention becomes clear. He is planning to swing his Rook over to the suddenly opened up h-file, advance his Kingside pawns and along the way recapture the now isolated White h6 pawn.
The rest of the game sees Keres advancing the typical KID avalanche of Kingside pawns and swinging over all his pieces to directly attack the White King.