Burnout Billboards are large red flashing billboards spread around the map, drive through them to smash them. Any you smash will show as smashed once you’ve done them, there are 120 in the game so you only need to do half.
Burnout Billboards look like this
Here’s a map with the location of all the Burnout Billboards
Smashing 60 will also unlock the following trophies