Basically, a harddrive stores the info magnetically on a spinning disk called a platter.
SSD's uses flash storrage. Basically like a USB drive, but faster because of different chips and controllers.
Problem with harddrives is the moving parts. It takes time for the physical arm to reach it's location. The arm simple isn't fast enough, and that goes for the rotation too. It can't compete with SSD's.
And to add to that, programs like chrome might have to open over 100 files spread around many different places on the platter. And it can't multitask. It can't read 2 files at once.
So that makes the harddrive slow.
SSD's have no physical moving parts and it's able to multitask.
Flash memory is a wide term. Can both mean SSD's USB's, SD cards and so on.