I hear it a lot. I used to think I knew what it meant in gaming. But I'm not sure anymore.

When it comes to Elder Scrolls and Fallout games, the long time fans tend to care really, really deeply about the lore. More than Bethesda does by a long shot, unfortunately.

I've noticed the "easter egg" title being called upon in lore discussions.
As in, one person will say "This is canon, this is lore," and another will say, "no, that's an easter egg."

So for me, lore is the enclave being able to make non-talking beasts into talking beasts, and it's aliens and their weapons, it's skynet, and it's even star trek phazers.
My first fallout was fallout 2, so I guess it set in my mind what the fallout universe is or can be, as opposed to Mad Max or the Walking Dead or another post-apocalypse license that might not have these things, or might have something else entirely. I just took it for granted that, in fallout universe, "SCIENCE!" does a lot of crazy things that "science" probably can't do.

For some, I've been told, "no, those aren't lore, those aren't canon, they're easter eggs."
I killed some plot-important characters with what appeared to be star trek phaser and then an alien blaster, and a vanilla companion was a talking deathclaw... so I don't know what the term "easter egg" is anymore.