Hola.
Mobility basically means how heavy a weapon is. The Feline for example is a pretty light weapon, you will be running faster if you are using it.
By adding attachments (scopes, extended magazines, hologram decoy) the weapon will become heavier and mobility will decrease/go down. Take MK 60 MOD for example, this is a pretty heavy weapon so when using it you will walk a lot slower, and if you add attachments you will walk even slower.
The higher mobility is the faster you will be.
The lower mobility is the slower you will be.