An SSD is a storage device known for its fast read and write speeds, meaning the time it takes for your game to load files will be significantly quicker. For example, if you are experiencing lag spikes a result of slow loading of in-game content, including maps - an SSD will most definitely help.
If you are hoping to improve FPS, unfortunately, it will not help much - if at all. You see, FPS stands for Frames Per Second and each frame is rendered by the system's GPU. The GPU is in charge of rendering audio, visual and animations which are then sent to your eyeballs for your viewing enjoyment. Sadly, the SSD has no effect on the actual display aspect of your game and so you will not experience better FPS.