「お待たせしました」 is a polite apology used when you have kept someone waiting. It translates to "Sorry to have kept you waiting" and is commonly used upon arriving, or when a task you were doing for someone is finished.

相手を待たせてしまったことに対する丁寧な謝罪の言葉です。到着時や、相手が待っていたことが終わった時に使います。