。
Yumi probably won't play tennis.
Literal
Yumi [topic-は] tennis [object-を] do-not-[conjectural-でしょう].
しないでしょう is the negative conjectural: 'probably won't do.' Chain: する → しない (negative) → でしょう (probably). A clean paradigm example of combining polite-conjectural でしょう with a plain negative verb form.