Multiply layer sets everything transparent :] If you make a Multiply layer on top of a Lineart layer, you can add colors without having it to erase the drawing from the Lineart layer. So you can color and still see lines!
Normal layer is kinda of like a digital version of a paper/canvas. If you draw something on a Normal layer and then try to color, you'll notice the color will erase or overlap the drawing (unless you're very careful). That's where the Multiply layer comes in very handy ^^
Overlay layer acts as a color dodge tool. It makes the colors increasingly bright, almost neon-like. Its very ideal for creating highlights or shine ^^