A fork is when you are able to threaten two pieces at the same time with ONE of your pieces. Knights are notorious for being able to do this, especially against rooks. For an example, you have your knight forking two rooks. Does it matter which of the two rooks you capture? Are you sure? Should you capture one of the forked pieces immediately?
These are the types of questions that you need to know the answers to if you want to consistently win games. When to capture a forked piece depends on what your opponent does with them. If your opponent leaves the pieces sitting there you can simply wait. If your opponent moves one of the two forked pieces then the answer is simple. Take the piece that you still threaten and take it now. But what if your opponent threatens your forking piece?
Using the example mentioned, let us suppose that your knight is now under attack and you must act. Which of the two rooks do you capture? The answer is to capture the rook that is doing the least on the board. If one of the rooks is simply stuck on the back row and the other rook is pinning one of your pieces, then capture the rook stuck on the back row. This will result in your opponent taking your knight with the other rook, thus unpinning your pinned piece, or it will result in you getting a "free" capture because your opponent simply allows you to take the rook. Always take the piece that is doing the least amount of work and always wait to capture until you are forced to capture, or until your opponent removes one of the forked pieces.