If you click the 'hot' tab on a member's (enlarged) picture, you're helping other members find hot material and giving that guy a compliment, (anonymously at the moment, but we may give you the option to let him know you think his pic is hot, in a future update).
The hot clicks he receives are shown in his statistics (see main menu), and are used to decide what gets shown in the 'hot pics' section on the HOME page, and the hot pics page.