XP Thresholds in D&D 5th Edition13 Feb 2015
In the 5th Edition Dungeon Master's Guide, XP thresholds are used by the DM to calculate encounter size (or perhaps appropriateness is a better word). You figure out the encounter difficulty you want, add each player character's threshold based on level and difficulty, then go shopping for monsters in the Monster Manual that meet this vague total, adjusting again for appropriate CR (challenge rating) since in general the CR should not exceed the party's average level. Once you have your stable of monsters, you do some more quick math based on the number of monsters likely to be encountered at one time in order to account for the greater number of attacks possible by larger groups. (Older versions of D&D used the term "No. Appearing" or "Number Appearing" which I always found to be a spectacular failure of terminology).
Most things in D&D 5th Edition appear very predictable and internally consistent. As much as I wanted it to not be true, charting hit point growth revealed a disappointly predictable set of lines, even though I simulated 20 characters from each class rolling for hit points after each level up, all the way to level 20.
With XP thresholds, there are no dice rolls to introduce randomness into the chart. But look! It's kind of a bumpy, satisfyingly weird chart. At first glance. It's only when you compare the bumps to character levels where specific ability score improvements or feature activations are likely to occur that this chart takes on the same consistency as the others.
One other note: looks like easy encounters are always supposed to be easy, eh?