Meaning of “ChikaIdol(地下アイドル)”

“ChikaIdol(地下アイドル)” means idols who are not signed with major agencies and mainly perform live shows.
“Chika(地下)” means underground. Some live house is in underground and also their activities are underground.

Unlike idols affiliated with major agencies, their activities are on a smaller scale, and they have a much closer relationship with their fans.
Not all of them, but some “ChikaIdol(地下アイドル)” seem to offer fan service that involves very close interaction—possibly with a slightly sexual undertone.
For that reason, the term “ChikaIdol(地下アイドル)” can sometimes evoke slightly indecent or suggestive images. (Of course, not all “ChikaIdol(地下アイドル)” are like that.)

Example Conversation

A: What do you usually do on the weekends?
B: I go see my favorite ChikaIdol(地下アイドル)!
A: Oh, really?
B: Yeah, she even remembers my name now!

A: 週末っていつも何してるの？
B: 推しのAに会いに行ってるよ！
A: そうなんだ！
B: もう名前も覚えてもらってるんだよ！