XP (eXperience Points) are used in the same way as many other games to mark the level a player has reached. You start the game at level 1 with 0 XP and move up to the next level when a certain number of XP have been gained.
In Mars Frontier you can gain XP by
- collecting resources from your Harvesters
- stealing resources from rival Harvesters and Storage Buildings
- completing construction or upgrades on any Building
- completing research on any unit
Note: You do not gain XP by collecting from quarries.
The level a player is at is a useful indicator of how advanced within the game they are, although there is a wide range of unit and building levels that can be achieved, depending on the path a player chooses to take when selecting which upgrades to perform first. The amount of possible divergence increases as the game progresses, although there will come a point when all researches and building upgrades are complete and player's upgrades at that level will converge.