Horde Chess sets an ordinary army against a wall of Pawns. Black has the usual sixteen pieces. White has thirty-six Pawns, no King, and nothing else.
White has no King, so White is never in check and never has to answer one. Any Pawn may be left where it stands, however it is attacked.
White's Pawns start on the first four ranks, plus b5, c5, f5 and g5. A Pawn standing on the first rank may step two squares, just as one on the second rank may - otherwise the back of the horde could never get moving.
That first-rank double step is not subject to en passant. A Pawn going from the first rank to the third passes the second, but Black may not capture it there. A Pawn stepping two from the second rank passes the third, and that one may be captured en passant as usual.
Everything else follows the normal Chess rules. White's Pawns promote on the eighth rank; Black castles as usual, and White never can.
Black cannot simply trade pieces off: every exchange leaves the horde one Pawn lighter but Black one piece poorer, and it is Black who runs out first. The pieces have to take Pawns for free, which means picking off the ones that step out of the mass and refusing to stand where two Pawns can hit the square.
White's strength is the phalanx. Pawns defending Pawns advance almost unanswerable, and a single passed Pawn escorted to the eighth rank ends the game. White's danger is the opposite: a hole in the wall lets a Rook in behind it, and Pawns cannot come back.