1. Literally, to quickly press on a vehicle's brakes to slow down or stop. He jammed the brakes on when he saw the child running into the street. You risk doing your car serious damage if you jam the brake on without shifting down the gears.

2. By extension, to slow down or stop something one is doing. In this usage, the thing being stopped can be stated after "on." We had to jam the brakes on development when we found out the investors were pulling out.