Lore-friendly means the same for mods that the phrase "canon compliant" means in fanfiction. It basically means that it:
1) Does not contradict the source material. If the source says character x was killed, contradicting that would be lore unfriendly. If character y is known to be a murderous thief, then characterising them as a good guy would be lore unfriendly. Another similar term in fanfiction would be Out Of Character (OOC). Making Harry Potter an emo metalhead who cuts himself, for example, would be OOC, or lore unfriendly. The same applies to altering characters/events in a game.
2) Is a logical extension of the source. For example, a mod that adds a medieval style dungeon under a castle would be lore-friendly because it fits in the setting of Oblivion (or Morrowind), as both borrow heavily from real medieval things for their setting. A mod that adds a disco strobe light/ball however, would not be a logical extension of the setting, it would be out of place to a very extreme degree. This would be lore unfriendly.
When a mod is lore friendly, it basically means it fits in with the rest of the commercially released ("vanilla") game, and doesn't stick out like a sore thumb. The term, like most terms of this sort, can be a bit subjective. As well, few modders bother to label or rate just how lore friendly/unfriendly their mod is because many don't care about it.
1) Does not contradict the source material. If the source says character x was killed, contradicting that would be lore unfriendly. If character y is known to be a murderous thief, then characterising them as a good guy would be lore unfriendly. Another similar term in fanfiction would be Out Of Character (OOC). Making Harry Potter an emo metalhead who cuts himself, for example, would be OOC, or lore unfriendly. The same applies to altering characters/events in a game.
2) Is a logical extension of the source. For example, a mod that adds a medieval style dungeon under a castle would be lore-friendly because it fits in the setting of Oblivion (or Morrowind), as both borrow heavily from real medieval things for their setting. A mod that adds a disco strobe light/ball however, would not be a logical extension of the setting, it would be out of place to a very extreme degree. This would be lore unfriendly.
When a mod is lore friendly, it basically means it fits in with the rest of the commercially released ("vanilla") game, and doesn't stick out like a sore thumb. The term, like most terms of this sort, can be a bit subjective. As well, few modders bother to label or rate just how lore friendly/unfriendly their mod is because many don't care about it.