Surfer slang (although also extends to other boardsports, and surf sports) for a young surfer. Can also be used to refer to children in general, although usually they have some association with surfing/etc.
Although usually there is no ill-meaning behind the term, it is often used to poke fun at younger surfers. It can sometimes be used as a derogatory term when younger surfers are annoying, bad or otherwise difficult.
It is frequently abbreviated to grom