Use the arrow keys to move and jump around the stage. There isn't any real goal, this is just me demonstrating how to make a platforming level with reliable code, the kind that could go into actual platforming games like Super Mario Bros. or something.
If you take a quick look at the platformer games on Scratch, you'll see that many of the praised ones will end up having common bugs and glitches such as clipping slightly through the floor, getting stuck on walls, or bouncing slightly on the floor. 'Thing is, it really isn't too difficult to overcome these, as long as you make sure to think about the problems that may come from the way you structure your code.