[RPG] the meaning of the term “natural 20”

"Natural" means an unmodified roll.

The number you see printed on the die when you just throw it. Not adding or subtracting bonuses, penalties or rerolling. Just the number you see.

Terms will differ in individual games and groups, but usually the total result (natural roll plus any modifiers) is just called your "roll," or we'll say "I got a 25." In some systems (sounds like you're reading a D&D book), your natural roll has implications regardless of the modifiers you can add to it, so it's important to have a way to talk about the difference.

"I rolled a 25 on a natural 1! My modifiers are awesome! ...but I still missed, because natural 1s always miss in our game."

"Man, I got a -2 on a natural 5. I need to get this curse removed as soon as possible or I'm gonna die of a stubbed toe."

(Five years on, my answer to this has changed a little; I'd avoid the word "traveler," too, since that's often been used to allude to the same peoples that are referenced by the original word.)

I think your instincts are good—the word harkens to stereotypes about real-world groups it's not helpful to promote. If you want to promote the idea of wandering performers with magical skill and some combat ability, you could go with "vagabond" or "mountebank" or something even more sinister like "charlatan." My recommended choice, though, would be the euphemistic "performer," which alludes to those evocative images without calling upon historical problems.