When you complete a track, the score earned is added to your XP. When you level up you can unlock a perk.
Perks are split into different classes: Scoring (Ways to get a higher score) Firepower (New weapons!) Firepower Boosts (More weapons) Survivability (More Lives) Other Difficulty
Unlocking Perks If you level up you'll be asked to unlock 1 or more perks. To start I recommend you scroll down and unlock: Goodie Bag (Get some pickups right at the start) Pump Up The Colour (Start with more progress toward activating the Beat Hazard beam) POW POW POW (Start with more progress toward activating the Beat Hazard beam) Gimme More Perks (Allows you to have more perks active at any time)
Enabling Perks Once unlocked, you need to go to the 'Perk' screen from the main menu. From here you can 'Enable/Disable' perks. Each perk can by upgraded using cash. Upgrade the 'Gimme More Perks' perk to allow you to have more perks enabled.
Perk Weapons There are 8 perk weapons to unlock. Of these 8, any 4 can be active. In the 'Fire Power' section of the Perk menu you can see which keys are assigned to your 4 weapons. You can then change which perk weapon is linked to your weapon keys.