These are alternative difficulty levels that add challenge by spawning tougher enemies earlier and more often. A Bandit doesn't get any tougher, but you may well see a Bandit Thug instead. It's a more interesting challenge than a simple damage multiplier. (Since this is intended to replace the in-game difficulty setting, it's recommended to have that setting at Adept.)

The inspiration for this was an article ( http://j-u-i-c-e.hubpages.com/hub/skyrim-difficulty-settings ) that suggested the difficulty settings should have adjusted level scaling instead of multiplying damage. These mods implement that idea by altering the generic scaling factors used in generating most leveled enemies. In the vanilla game, these factors range from 33% to 125%. With one of these mods, they are increased significantly.

The result is that leveled spawns often generate tougher enemy types than usual. Unaffected by the mod are those who level up directly with the player (e.g. Malyn Varen) and those with a fixed level (e.g. Volsung). Since leveled lists are only well-populated up to a certain point, this mod is most effective for characters up to a certain level--40 for Thane, 35 for Jarl, 30 for High King. Beyond that, the challenge will diminish as your character's abilities increase.

Each of the mods in this collection represents a different difficulty setting, as shown below. If you enable more than one at a time, only the lowest in your load order will take effect. The difficulty levels are as follows:

033% to 125% ---- Citizen (no mod enabled): No guts, no glory.
067% to 150% ---- Thane: A respectable challenge.
100% to 175% ---- Jarl: Worthy foes abound.
125% to 200% ---- High King: Sovngarde awaits.

Note that unlike the in-game difficulty setting, adding or removing an Immersive Difficulty Levels mod will not affect already-existing enemies. (If you can't finish a dungeon, disabling the mod won't help.)

Immersive Difficulty Levels can also be found on Skyrim Nexus at http://skyrim.nexusmods.com/mods/35569 .