This phrase is a polite way to decline an offer or suggestion. "いいえ" means "no," and "結構です" indicates that you are fine as you are, or you've had enough. It's more polite than a simple "いいえ" alone, conveying gratitude while declining.

相手(あいて)からの申し出(もうしで)や提案(ていあん)を丁寧(ていねい)に断る(ことわる)時(とき)に使う(つかう)ます。「いいえ」は「No」で、「結構(けっこう)です」は「もう十分(じゅうぶん)です」や「必要(ひつよう)ありません」という意味(いみ)です。