XP stands for Experience Points. You need XP to level up.
Almost everything you do on Bin Weevils will earn you XP. Planting and harvesting your bin garden and buying items for your nest are the best and easiest ways to earn XP.
Once you have earned enough XP to level up, the "My Nest" icon, in the bottom left hand corner of your screen, will begin to glow. When this happens, click the icon to return to your nest. You will hear fireworks and your weevil will start glowing. A sign will come down from the ceiling to congratulate you on having levelled up.
Don't forget to check your "My Stuff" chest for a new level trophy, each time you level up!
Currently, the maximum level for a weevil is 65 at approximately 350,000 - 400,000 experience points. However, Scribbles is currently on level 70, and it is said that all weevils will be able to get to that level because before the maximum level was 60, with Scribbles being 5 above us; at 65.
Release and Development of the experience feature
Experience points were originally released on 30th May 2008, a week and a half after the binpets were released. Bin pets had experience points BEFORE weevils did. Levels were not added until 2011, and the levels now work with the experience.