An entity's level is a relative descriptor of the entity's capabilities. Levels range from 1 to 10.
A PC's level determines base HP, EP, and WP values and what cards can be added to their decks. Various role abilities improve as levels rise. Many Gear cards have level requirements in order to equip them; this is particularly common with Enchantments, Rituals, Augments, and Implements. A PC's level must be equal to or greater than the level of the card or Gear they wish to use or equip.
The DM determines when PCs gain a level. This usually occurs at the end of a lengthy adventure or sequence of small adventures. A PC's level is used for all three roles; they do not increase independently of each other.
The various types of enemies PCs face also have levels. These are primarily used in order for the DM or adventure designer to select appropriate enemies to use when constructing a scenario.
For more information, see Player's Guide Chapter 2: Roles and Levels.