XP means Experience Points and you collect XP to level up! Each level will require more XP than the previous level and you can see your progress towards the next level shown by the progress bar in the upper left corner. Your level will be displayed in the star icon and then your progress will be indicated in yellow as you fill up the progress bar. Feeding and releasing species will earn you XP and you will also earn XP when you Establish a species and learn the fact for it! Different species give different amounts of XP. You can see the specific XP totals shown directly under the progress bar when you earn XP.