Gear decrypted from engrams can have multiple perks.
Common (White) gear gets 1 perk.
Uncommon (Green) gear gets 2 perks with slightly higher potential than Common (White) gear.
Rare (Blue) gear gets 4 perks.
Most Legendary (Purple) gear gets 6 perks.
Exotic (Yellow) gear gets 8 perks.
The perks that are possible include: HP, Attack, Defense, Glimmer, XP, Location Point, Critical Hit Chance, and Evade Chance. Each of these acts as a percentage boost for your Guardian. Gear with perks will show the perks and their values in the description of the gear within your gear changing menu.
Light represents your overall power in the game. It is based on your Guardian's overall traits, K/D ratio (miniscule) and your gear. The higher your light level, the better gear you can acquire from decrypting engrams.