Legal’s Mate is a checkmating pattern involving two Knights and one Bishop. It’s achieved typically in openings when your opponents Bishop is undefended.
This mate is commonly seen in the openings of beginner Chess players. Even though only a few moves have occurred, checkmate is always a threat to each side.
This example is from Kermur Sire De Legal himself when he first used this mate in 1750. Legal’s Mate was born when this game ended.
When white plays Nxe5, Black is in a dangerous position. However, most beginners get too excited when they see a hanging piece, especially the Queen. Black takes the Queen and at the same time, losing the game.
Try to remember to not get too excited at any one move and carefully analyze the position after every move. Is Black accepted losing a pawn and played dxe5, the game would have continued.
For more information on the names of the Squares, read the guide on Chess notation.