This is David from Chess NYC, here is the king. The King is a very simple but important piece. He moves 1 square in any direction, however, the king, because he's the most important piece, always has to stay safe. So let's say black has a rook here, on H4, the king cannot move to any square in line with the rook because then he would be in danger of the rook capturing the king. This is called moving into check.
If a piece attacks the king, he's in check and has to get out check. The king is not allowed to voluntarily move into check. If, for instance, black had a rook here, and the queen here, this rook on B1 confines the white king, he cannot move out here because he'd be under attack from the rook. So let's say he makes 1 of his moves, he can either go down 1 square or up 1 square, then black could bring the queen over here to A2, checkmating the white king, the queen is attacking the king. check, the king can't go more than 1 square at a time, so he can't go 4 squares at a time, capturing the queen, there's nobody that can block this attack, if let's say white had a rook, he can bring the rook over to block the queen's attack on the king but he has no rook. If he tries running out here, to the B5, he'll be under attack from this rook, so he can't go out here. Game over, check mate.
Most important thing in chess, keep your king safe.