Traditionally hard drives are spinning device that is non-volatile storage, which means the data is not lost when the power is turned off. A hard drive is a metal platter that is covered with a magnetic coating to store data. Do you need data? The read/write head on an arm accesses the data while the metal platter is spinning. The faster the platter spins, faster the HDD performs.
But SSD (Solid State Drives) stores data on an interconnected flash memory chips and does not lose data when the power is turned off. Flash memory chips are faster and more reliable. It means SSD disk does not spin.