It's a thing with small holes in it to let hot water through to steep (brew) tea and then can easily pull the leaves out of the cup so you can drink the tea without getting the leaves stuck in your teeth.
Don't use the balls. They are a pain and easy to over stuff where they don't steep well.
Ah. That dang thing I have been looking for at least a week at home! We had several of them and I can't find any! So I use the kettle for French press coffee from Ikea for tea. It works pretty well. https://www.ikea.com/cz/cs/p/upphetta-konvice-na-caj-kavu-sklo-nerezavejici-ocel-kremova-80583739/
That's an interesting option. The only issue I have with that is yes another thing to wash up.
True. But it is surprisingly easy. Just pour a water from tap and it's clear. No issue there. At least not for the tea I'm currently drinking.