The Proton Nano Mini Marker is used to mark the lie of your disc when you're playing a round of disc golf. You don't need it for casual play, but if you're playing in any sort of organized tournament from national tour all the way down to random doubles you're going to need one.
They're useful for other things as well. Upside down minis make great coasters, and they're lots of fun to throw as well. They're available for a pretty good price at Sabattus Disc Golf, because we know you may want more than one if they run out of luck.
These really are in proton plastic from MVP so they're high quality, durable, and you can see through them.