Yerba Mate is a South American plant. The leaves can be dried and brewed in boiling water like tea to make a beverage also commonly called Yerba Mate.
The traditional brewing style uses a larger wooden vessel and a long straw with a filter on the submerged end. Yerba doesn't get bitter with prolonged steeping like tea does, and you can resteep so it's common to keep adding new boiling water to the cup and get 2 or 3 (or more) brews. But you can brew it like regular tea also.
Yerba has a lot more caffeine than black tea, but I find I'm less jittery and have a smoother fall off than with tea. The taste is unique, kind of "green" and light, slightly bitter. I like a bit of honey in mine. The company i usually buy tea from also offers Yerba and guayusa blends (like masala chai style, roasted, other flavors) and I really enjoy them. Guayusa is another plant that has natural caffeine.
I highly recommend Yerba! it's a very pleasant buzz and a nice alternative to the usual teas and coffees.
Addition:
I buy from NMTea Co in the USA. This isn't an ad or anything, but I like them and regularly buy from them. They also sell the equipment for traditional Yerba if you want to do it right. (I just use my tea stuff like a heathen, haha)