XP, as many of you well know, stands for "Experience Points," and simply denotes how much you've used Salad on a given account. To find XP, navigate to the "Earn" tab on your app and select "Summary".
Here you can find your XP value and the Veggie Badges that you've earned in The Pantry.
The longer you run Salad, the more XP you accrue and the more Veggie Badges you'll unlock. During Seasons, you can also earn Season XP which will unlock you bonus rewards. You can read more about Seasons and Season XP here.