Level is the primary characteristic that determines a Guardian's progression and power.
In Destiny, a Guardian can reach a maximum level of 40 by gaining experience. After reaching level 40, a Guardian can increase his or her power increasing their Light.
In Destiny 2, a Guardian can reach a maximum level of 20 without DLC, level 25 with Curse of Osiris or Warmind and level 30 with both expansions by gaining experience.