In destiny 2 there are two different leveling systems and as you progress through the game the amount of power you possess becomes more of a factor. Your character's Power Level will depend on what equipment you have equipped. The more powerful, and the rarer your equipment, the higher your Power Level is. At launch, Destiny 2's max Power Level is 350.
To reach this maximum Level will involve spending plenty of time completing quests and Raids as these will get you Legendary and exotic Engrams that you can use to craft more powerful items. You can also get more power through Milestone as they award enhanced equipment that has a higher Power Level than the equipment you have equipped when you complete them. It is important though that you don't use Milestones to boost your Power Level and equipment until your power level is high enough that you will get valuable equipment out of them.