Huh? What's that? Are they doing some kind of performance?
Literal
Huh? What [copula-だろ]? Performance [even-でも] doing [question-のか]?
あれ is the surprised interjection for noticing something unexpected. 何だろ is the casual wondering form of 何だろう 'what could it be.' パフォーマンス is the loanword for 'street performance / show.' でも here means 'or something like that.' A natural, casual 'what's going on over there?' observation.