Mobility is the ability to move freely. If your basketball injury causes you to lose mobility in your knee, that means you can’t move it very well.
Mobility often refers to whether you can move an injured body part, like a joint or a limb, but it can also describe movement in general. If you have great mobility on the tennis court, that means you move freely and easily, running down shots with ease. Mobility can also describe movement between different social or economic levels. Your fancy new job may offer opportunities for upward mobility.