Light-level

A guardians light-level is a measure of her power. To increase this power a guardian must fight through hordes of enemies to improve her equipment. The game states that your light-level is the average of all the gear, which is true with modifications.

Each weapon contributes 12% to the average, each armor piece 10% and the ghost contributes 8%.

Bear in mind that all the following numbers are for PvE. The guardians light-level determines the damage done with abilities i.e. grenades, melee and her super. In dealing damage with weapons light-level has no direct effect. However there is a penalty to damage if the guardians light-level is underneath whats recommended. By having a negative difference of five light-levels you incur a penalty of 14.5%. The full table with more details can be found in the appendix.

This also means that you want to have a weapon with the highest attack possible in most situations. For each level of attack you increase there is somewhere around 0.7% damage increase. The discussion of light vs. perks vs. weapon is a fair bit more nuanced. More info on the topic is found in the weapons section.