An experience adjustment is listed with some of the species. This is a reflection on the capabilities of the species. If a species is more physically or mentally capable, then they don't have to work as hard to accomplish their goals, and therefore will earn experience at a slightly slower rate. The species that are at a disadvantage will rely on earned experience more than the capabilities they were born with so they will earn experience at a slightly faster rate.
There are two ways that this adjustment can be used. Obviously, the first is when awarding experience to a character. When a character has earned ten experience, you would add the experience adjustment to the ten experience. In the case of a negative experience adjustment, the ten experience will go down and thus, the character will need to earn more experience to reach the next stage. The negative adjustment would only be added once per stage. In the case of a positive experience adjustment, the character would have experience added and thus have a head start on the next stage.
The second way the adjustment can be used is during character generation. Often when creating a character you start them off at second stage, or third stage, etc. But, you could add some variability to it. Let's say you want to start the character in second stage. You could say that their experience is 25 plus the species experience adjustment. This would keep them in second stage, but would make the capable species work a little harder to reach third. To add even more variability, you could add a dice roll. For example, roll 1D6, where a 4, 5, or 6 is an addition of 1, 2, or 3 experience respectively and 3, 2, or 1 is a negative of 1, 2, or 3 respectively. If you add the experience adjustment, it is possible that the character could cross a stage boundary in either direction.
Obviously, these are optional rules. They are used when the referee wants to inspire the players to pick species other than Groten, Massetin, etc.