Once you completely liberate a province it reveals all of the collectables and Daredevil Jumps in that region. Daredevil jumps are the symbols that look like a guy on a bike in either white or blue (see below). White symbols are ones you've not done yet and blue ones have been.
There are 30 in total and if you're struggling to spot one then zoom out in the map and a box in the bottom left corner will tell you what you've unlocked or collected in each province.
You simply need to drive off each one with any land vehicle and as you do you'll see a counter in the right side of the screen tick over after you're in the air for a second. You don't need to jump far or even survive the jump, just get a vehicle over there.
When you do this you'll also unlock a new bike which is needed if you want every vehicle in the game.
The bike you unlock is not needed for the every vehicle achievement, thank you Matrees.