Frame rates are a technical term for how often your monitor refreshes. For example, 60 FPS means your monitor is refreshing 60 times a second. The lower the FPS, the choppier the game and animations will look. The higher the FPS, the more fluid the game will look. Generally, you want to have at least 30fps - more for high movement games.
A frame rate cap puts a maximum on the frame rate for the game. They are a built-in game setting - not something you have to pay for. Higher frame rates use more system resources since the monitor is refreshing more times a second. That is why sometimes people will choose to set a cap. At higher frame rates, it is very difficult to discern any differences, so it may not be necessary anyways.