Grand Theft Auto III - The Definitive Edition

Each island has several unique jumps that require you to get in a fast car, hit the jump at the right angle and land in the right area. It’s recommended that you use something like the Banshee as most jumps you can't over jump, then get a good run up before hitting the ramp and landing in the right zone. 

Completing all 20 jumps will reward you with $1,000,000 as well as being required for 100% completion. The GTA wiki has an excellent guide for locations or follow the video below.