A hover state is how an item looks while a mouse pointer is resting over it. It is like a door handle changing color when your hand reaches for it, hinting that it can be used.
A link might change color or a card might lift slightly when the pointer moves onto it. This feedback can help people notice which item they are about to select.
Hover only helps people with a pointer. Important actions and information must also work for touchscreens and keyboard users, who may not have a hover state at all.