This is a polite phrase used when you didn't catch what someone said and want them to repeat it. "すみません" means "excuse me," and "もう一度" means "once more." "お願いします" means "please." It's very useful in conversations when you need clarification.

何(なに)かをもう一度(いちど)言ってほしい時(とき)に使(つか)う丁寧(ていねい)な表現(ひょうげん)です。「すみません」は「ごめんなさい」で、「もう一度(いちど)」は「もう一回(いっかい)」という意味(いみ)です。