What is a checkmate?

A checkmate is when a king is in check, and it is impossible to get out of check.

Think of it similar to taking other pieces. If the king is at risk of being taken by another piece, it is in check. You can't actually capture the king, but it's the threat of it.

Make sense? I can explain it differently if needed.

Yes, a king is in check when another piece is attacking it. If you cannot block that check, capture the piece attacking the king, or move the king out of check, it is checkmate. Also, if the king has no possible squares to move to (Because it is an illegal move to put your king into check), then it is a stalemate.

When the king is in check another piece is threatening it. In this situation you have to get out of check. If its checkmate then the king cannot get out of check and you cannot block with another piece.

A Level 1 threat that cannot be met.

When the king is in check, and cannot get out of check.

Checkmate is any game position in chess and other chess-like games in which a player's king is in check and there is no possible escape. Checkmating the opponent wins the game. In chess, the king is never actually captured—the player loses as soon as the player's king is checkmated. 

To mend this stalemate statement, this is only the case if all other remaining pieces are blocked or pinned.

Checkmate is when the king is put in a spot with enemy forces putting their swords to his throat andd holding him down, like if the king is in a forest with archers and swordwielders pinning him to a tree, or against a wall in his castle, with spears pointing at his person with no way to escape,

A check would be "Uh-oh, that archer got me in his sight, I better move or I'm dead"

This is a checkmate play. Look at the move with the "#" symbol. That is the checkmate move.

Here's another one.

Checkmate just means that the king is in check and there is no way for the king to escape, it is one of the ways you can win a game