。
She's having an affair with her boss.
Literal
She [topic-は] superior [with-と] affair [object-を] is-doing.
浮気 (うわき) means 'affair, infidelity, cheating' — literally 'floating feelings.' The をしている form marks the affair as an ongoing situation. 上役 (うわやく, 'superior, boss') is a formal term for someone higher in the workplace hierarchy, more specific than 上司 (じょうし) in implying direct authority.