有劳 is a polite fixed expression used to request a favor or to thank someone for having gone out of their way to help you. It literally means 'to have troubled (someone)', and it's more formal than 麻烦 or 请. You can use it at the beginning of a request (有劳你帮我一个忙) or after someone has done something for you (这件事有劳你了). The tone is respectful and slightly deferential, suitable for service staff, elders, or colleagues in formal settings.