A fork is a move in which a piece or pawn attacks two enemy pieces or pawns at the same time.
Above we see examples of forks by a pawn, queen, rook and the king.
and forks by the bishop and "king of forks" knights.
After juniors learn to look after their pieces by not leaving them hanging (on a square where they are attacked with no protection). The one move fork is the next challenge to keeping those pieces safe.
The following are positions from my early games (as a 30 something).
In this game white has let black's knight start wrecking his position and panics with bishop takes bishop.
Black (me again) has just greedily taken on e4.
Its not just the juniors that have to watch for simple forks...