SSD stands for Solid State Drives. SSD drives are much faster than traditional hard drives. When it comes to server performance, they make a huge difference. SSD drives contain no moving parts, whereas other types of hard drives (like HDD) rely on a mechanical arm and a spinning disc to read and write information.
SSD drives can read and write files much quicker. Web servers especially spend most of their time reading and writing files. It is in a way, their main function. So using SSD drives instead of (cheaper) more traditional hardware can speed up site performance by 100%!