Wheel durometer, or hardness, is generally rated on a scale called “Durometer A Scale” with hardness ratings from 1-100A, 100A being the hardest.
Softer Skateboard Wheels (<78a to 95a) are great for skateboarding on rough surfaces. They aren’t appropriate for technical or street skateboarding, as the grip and softness of the wheel reduces the ability to pop the board off the ground and perform tricks (especially grinds & slides). These wheels are frequently used on boards meant for easy cruising, or by filmers that want a smoother, more stable ride.
Harder Skateboard Wheels (96a to 100a) are more suited to technical street & park skating, offering greater response and rebound, giving skaters the ability to slide the wheel/s along surfaces to perform various tricks.