These achievements require you to jump over obstacles. "Jumper" is awarded for jumping over one for the first time, while "Athletic Training" requires you to jump over 20 obstacles in the same campaign mission or multiplayer match.
At the very beginning of the first campaign mission, as soon as you get off the boat, there will be a small fence of sorts in front of you to the left. Repeatedly jump over this fence until you reach 20. Obstacles are climbed by pressing the sprint button (just below and to the left of the shoot button).