Master this and many more motifs
Forks are the most common tactics that win material. With a fork you make a move with one of your pieces. After the move, this piece is attacking two (or more) of your opponent''s pieces at the same time. Very often, your opponent won''t be able to defend against both threats with the next move, and you can capture one of the attacked pieces.
When you start learning forks, you have to look at all possible moves to determine which pieces are going to be attacked by that move. With more and more practice, you will be able to quickly see which attacking moves are possible, and you will spot many forks immediately and without effort. Some forks can be more difficult to see, if they involve the whole board, diagonal backward moves or attacks, or in complicated positions where a lot of moves and attacks are possible. If there are any unprotected pieces, forks (as well as some other tactics and combinations) are much more likely. Therefore, it is always a good idea to look for unprotected pieces and any way to attack those.
About this level
You have already mastered knight forks, pawn forks, rook forks, bishop forks, and queen forks. In this level, all of the forks are possible, including king forks which are the rarest, and don't have their own level.
White can win a piece with the king fork Kc4, attacking both the knight on b5 and the bishop on b4 at the same time.