Experience/XP
Earning enough experience will grant you an ability point that you can spend on the Abilities tab of the Upgrades section of the pause menu in-game. You can check this menu to see how close you are to earning another ability point.
Power/P
Earning enough power will unlock a new row of abilities to use ability points on. You can view your progress towards the next row of abilities by viewing the Upgrades section of the pause menu in-game.
Mirian
Mirian can be spent on the Attributes tab of the Upgrades section of the pause menu in-game. This allows you to increase your health, add additional rune slots to your weapons, and more.