Guardian swag levels are a levelling system featured in Destiny.
They are earned every time you level up. When you level up you will receive three swag coins, these can then be taken to the Cryptarch and be exchanged for swag points. These three coins will grant you enough swag XP to level up once.
Swag levels are used to buy weapons from the vendor and can be used to earn more vanguard points.
They were not featured in the Beta, but are expected in full game.
Not much else is known about swag coins.