What is welding electrode coating do?

It melts. Avoid moisture. To cover the weld. ------ Plus adds alloying ingredients, determines the type of electrical current to be used (AC - DC or either), slows the cooling rate of the puddle to temper the weld, tells you what position you can weld in (flat,horiz,vert,overhead), protects the molten puddle from harmful gasses in the air by creating smoke and slag, determines how much amperage you will need to use, shapes and contours the weld bead and much more.