Shell Jumps are a basic kaizo trick where Mario bounces off of a shell in midair to gain extra height. It has many variations such as Double Shell Jumps and Midair Shell Jumps. The basic shell jump involves throwing a shell into a wall so that it rebounds, landing under Mario.
For a standing shell jump: Stand 2-3 blocks away from a wall. Hold Y to grab the shell and then jump with B. Near the peak of your jump, quickly release Y and then press it again. Then press right to get over the wall. If the shell goes through Mario, you were too close to the wall. If the shell goes under you, you threw too early. If it kills you, you threw too late. If you do a shell jump farther away from the wall than usual, you will have to adjust your timing slightly earlier.