A mooncup is a brand of menstrual cup.
A menstrual cup is a reusable menstrual product. It’s a small cup, made of medical grade silicone, which sits inside the vagina and collects the menstrual blood. It can then be emptied and re-inserted. They can last for, if I remember correctly, around 5 years.

It can look slightly…intimidating, but when you insert it it’s folded up. I highly recommend it, especially if you’re already using tampons, as it’s much safer (they’ve been around since the 1920’s and there has not been one case of TSS associated with menstrual cups) cheaper, and comfier.