Xp (experience point) are points that are used to level up. These can be earned in different ways.
Battles are the main source of xp. You get xp by winning or for destroyed modules if you lose. The amount of xp gained depends on the enemy ship.
Fleet arena is a rather expensive way of making credits. The amount of xp earned is determined by the amount and level of the ships you destroy and lose.
Most Daily Tasks contain a certain amount of xp. You can collect them once a day.
Sectors oftenly contain xp rewards for winning battles and completing the sectors.