I've done a few skydives so I can speak from experience - the rig that your canopy / emergency reserve is attached to resembles a giant backpack with a chest strap, and two straps that go around your thighs, and when you deploy the main canopy, you will feel the rig exerting a braking force on your chest, shoulders, and thighs. It's a lot like slamming on the brakes in your car at 60mph+. If your jeans are too tight-fitting you may get a rash, or it may just be really uncomfortable.