What is a “locator” for finding rentals in Houston?

Context: moving to Houston this summer and have been looking for rentals. This is my first time living in a larger city so I have a couple questions that are unfamiliar to me…

• a lot of the rentals have “locators” or people who help find apartments? Is this a scam? Who pays their fee?
• if I’m looking to move end of July or August 1, when do I start looking?

Thank you all!!!