。
She slowly moved forward.
Literal
[topic-は] slowly advanced.
前進する ('to advance, move forward') is a Sino-Japanese compound: 前 ('front') + 進 ('progress, proceed') — together 'forward-progress.' Pairs with する as a する-verb. ゆっくりと modifies the verb adverbially with the optional と connector. The Sino-Japanese choice gives this sentence a somewhat formal flavor — in casual speech, 前に進んだ would be more natural.