。
Her English ability grew rapidly.
Literal
She [topic-は] rapidly English-ability [subject-が] grew.
A classic 'double-subject' construction. 彼女は establishes the topic ('as for her') and 英語力が is the grammatical subject of the actual predicate 伸びた ('extended, grew'). Japanese routinely uses this pattern when a property of the topic is the real subject of the verb — 象は鼻が長い ('elephants have long trunks,' literally 'as for elephants, the nose is long'). 英語力 itself is a productive Sino-Japanese compound: 英語 ('English language') + 力 ('power, ability'), and the same 力 suffix produces 漢字力, 集中力, 想像力. 急速に is the adverbial form of the na-adjective 急速 ('rapid'), more formal than the everyday 急に.