What's a "Paid Protester"?

I’m going to a protest in my area and will be contributing. My mom’s boyfriend said that it will be dangerous to go because there is a notorious “Paid protester”. The place where the protest is happening is used to being peaceful, the police are there but do not intervene unless physical violence occurs. This protest will be an expression of free speech and is intending to be a PG event according to flyers.