This is a simple game paddle, moves left or right depending on whether you press the left or right arrow keys. You can also control it's speed by pressing the up or down arrow keys. I've added a bunch of comments to help walk you through what each block is doing. So after clicking "See inside," start with the comment that begins "START READING HERE." Hint, it's in the top left of the code.
You can see a variation of this paddle used in the game Breakout here: https://scratch.mit.edu/projects/370291352/