Hi Sallender and welcome to GoY. Mulch is a layer of material applied to the surface of an area of soil. It is used to conserve moisture, suppress weeds and to add fertility to the soil. It can be all sorts of things, composted bark, spent mushroom compost, well rotted manure, compost, wood chips and leaf mould are all examples of organic mulches.

I agree with all that Mg has told you. I will add that different mulches can alter the ph of your soil. Mushroom compost will make it more alkaline and bark will make it more acid.

Here in the desert, we frequently use gravel as a mulch, which helps with weed suppression and water conservation, but doesn't do a thing for soil cooling or quality. Since it normally is used in dry gardens with native desert plants--which are used to poor, hot soil--that's normally not a problem. I can't recommend it for roses or veggies, though.