For a variety of reasons, XP in Webs is measured in encounter units, or “E”. That is, 1E is equal to the amount of XP a typical party would gain from a baseline encounter. Per DMG, this is one-tenth of the XP needed to level.
For example, it takes 1000XP to for a single 1st level PC to reach 2nd level. Thus a typical party of five requires 5000XP. One-tenth of this is 500XP, so 1E (at first level) is 500XP.
Put another way, after 10E has been awarded, the party levels.
The value of E changes with the level of the party. For a 2nd level party, E = 625XP. No matter the the level of the party, 10E is always one-tenth of a level.
E notation makes it easier to visualize pacing when writing campaigns.