。
She felt someone touch her back.
Literal
She [topic-は] someone [subject-が] back [object-を] touch [the act-のを] felt.
誰か ('someone') is the indefinite pronoun built from 誰 + か — the same pattern as 何か ('something'), どこか ('somewhere'), いつか ('someday'). The clause 誰かが背中をさわる ('someone touches her back') is nominalized by の and marked as the object of 感じた with を, giving the standard 'feel something happening' frame: [event clause]+のを+感じる/見る/聞く. The が inside the embedded clause marks 誰か as the doer of the touching, while the outer は keeps 彼女 as the topic of the whole sentence. A neat illustration of how Japanese can stack two subjects (の-marked and が-marked) in a single sentence.