Island Billboards are large orange flashing billboards spread around the island, drive through them to smash them. Any you smash will show as smashed once you’ve done them, there are 45 in total so smashing 15 will be a breeze.
Island Billboards look like this
Heres a map of all 45 Island Billboards
Here's a link to Mario87PL's youtube channel with a video for all 45 Island Billboards