。
She slipped quietly out of the room.
Literal
She [topic-は] quietly room [from-から] slipped-out.
そっと is a 擬態語 mimetic for hushed, careful, inconspicuous action — moving without making a fuss. Despite looking like an onomatopoeia, it doesn't represent a sound but a manner. 抜け出す is a compound verb pairing 抜ける ('slip through, escape') with 出す ('exit, put out'), giving 'slip out, sneak away' — usually with the connotation of leaving discreetly or unauthorized. The combination of そっと + 抜け出す paints a vivid mini-scene of stealthy departure, the sort of phrase a learner might recognize from a novel or a movie scene.