This phrase is used to politely ask someone to speak or do something more slowly. "ゆっくり" means "slowly." It's very helpful for beginners learning Japanese when native speakers might be speaking too quickly to follow. You can add "すみません" for extra politeness.

相手(あいて)にゆっくり話(はな)したり、何(なに)かをゆっくりしてほしい時(とき)に使う(つかう)丁寧(ていねい)な表現(ひょうげん)です。「ゆっくり」は「速(はや)くない」という意味(いみ)です。