The mate straw (called “bombilla”) goes into a cup (“mate”) full of hot water and yerba. Its job is to let you drink the yerba-flavored hot water while keeping the yerba out of your mouth.
The one I bought today had a metal circle around it. I’ve never used one with it before. The guy who made it says it’s there to make it easier to clean the used up yebra out of the mate when you’re done.
Unlike coffee which changes based on the ingredients you put in it, most of us who drink yerba mate basically drink the same thing, but it’s always personalized in more sutble ways. I have a way of angling the yerba before adding water which lets the flavor last longer. Others have a way of shaking the cup to keep the thin, dust-like particles from going up the straw.