These are dried Hibiscus calyces but depending on where you are, you might also know them as Sorrel, Jamaica, Roselle or Karkady. It's probably more common to find hibiscus used in fruit blends but it is worthwhile trying to find these whole flowers.
Hibiscus is high in Vitamin C, calcium, iron, niacin and riboflavin. It's even more beneficial property comes from the presence of Anthocyanin, a powerful antioxidant. Studies have shown that Hibiscus tea can help decrease the levels of bad cholesterol and decrease blood pressure.
To brew, the general rule is to use one calyces per person. You can also let it steep in either hot or cold water.
The resulting infusion is this gorgeous vivid red - it's slightly tart so I recommend adding a little sugar especially if you're making an iced tea.