Rush is a gear perk that raises your general move speed. It is a simple pick that helps you cross open ground, reach cover and get around Yara on foot more quickly.
You walk and run faster at all times. That shortens the time you spend exposed between cover.
Wear the Parkour Kicks in the feet slot. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Fine for early play. Once you have the Lightspeeders, Sound Barrier does the same job far better.