You start at level 1 of a class of your choosing, and advance in level as you progress through a campaign. Alternatively, you may join an existing campaign at a higher level. The maximum level a character can reach is 20.
You can gain a new level either through experience points or milestones. Once you gain enough XP or a milestone threshold is reached, you can gain a new level.
You have a pool of experience points ("XP"), which represents your success on achieving an objective, as well as how clever and successful you were at it. You start with 0 XP out of a maximum pool of 100. You can gain XP by:
Defeating Monsters. Once combat is finished, the GM grants XP equal to the company's level divided by the monsters average level.
Discovering Landmarks. Discovering a landmark grants XP equal to its level (once per landmark).
Finding Treasure. Looting an artifact grants XP equal to its level (once per artifact).
All the XP is then split evenly among PCs and their companions, either at the end of a session, an adventure, or a campaign. Once your XP pool reaches 100, you gain a new level, and the pool goes back to 0.
Alternatively, the GM might use milestones, which represents certain points during an adventure that you reach, granting you a new level. The GM decides which point this is and how many levels it grants. Usually, a milestone threshold is reached once a certain number of sessions have passed, an important event during or at the end of an adventure happens, or once a campaign is completed.
Once reached, your level in your class increases, granting you the benefits of that new level. This level also represents your character level, which increases or improves different statistics, such as training bonuses, and access to new features, such as talents and traits.