。
She is singing on stage.
Literal
She [topic-は] stage [on-で] is-singing.
A simple present-progressive: ~ている (here in the polite ~ています form) describes an action in progress. The で marks the location where the singing is taking place — location of action, distinct from the location of existence (に). Polite ~ます-form, used as the present continuous, is common when narrating live events, captioning pictures, or describing what's happening on screen. ステージ is an English loanword that competes with native 舞台 (ぶたい, 'stage'); ステージ leans more toward the modern entertainment-industry sense.