An advocate is someone trained to listen and help you to speak up. They are not usually legally trained. An advocate is a support to you and they will speak with you, or if you want, speak for you. Their loyalty is with you.
If you need help to make a complaint or to receive the services you need, then having an advocate who understands the system, and is there to support you, can be a great help.
The short video below describes how someone can speak on your behalf and help you to get your voice heard: