Experience is something that every physical object possesses innately. Inanimate and non-living things have the lowest experience level associated with them, while living beings have more.
Soil -> Minerals / Rock -> Plants -> Insects -> Small animals -> Large animals -> People
Conscious entities are aware of their experience in a way similar to how we are aware of the lack of pain in our body - it’s not something that is actively there but is noticeable when thought about directly. When a certain level of experience is reached, the person ‘levels up’ and becomes stronger through added stats which depend on the person’s main class. These stats are shown through a person’s strength or agility, while the actual numerical values are unknown to them.
The experience system is designed so the average villager is still able to reach a decent level through their day-to-day activities. For example, a farmer would gain experience from toiling the soil, cutting down trees and slaughtering livestock. They’ll never get to a level comparable to that of a soldier or mercenary, and therefore wouldn’t gain as many class-specific skills, but the general population is in no way useless compared to the elite.
Magic-users that specialise in defensive (or just generally not offensive) spells still gain experience by casting those spells. Their gains are quite high as most spells affect people or multiple targets, but because it can become quite draining they end up balanced with the usual soldiers.
Each level is roughly double the experience from the last and doesn’t carry over between classes, i.e if you specialised into a Sentinel after being a lv. 10 Defender, you would start as a level 1 Sentinel and keep the stats from the Defender. It takes roughly a year and half to move between levels. Therefore, it’s not uncommon for those who’ve stuck with their natural class their entire life to reach level 50+.