It’s a lynch pin.
It goes in that empty hole on your tractor that needs a pin to stop parts from coming apart. The hole is likely through a large bolt, stud or pin where other parts connect.
It’s a part that normally or occasionally is expected to come apart. That’s why they used a lunch pin instead of a bolt so you can take it apart without wrenches.
People (me) have sometimes put them through their holes facing the wrong direction. That is, the flat side of the pin should be against the object it is holding together. Otherwise the snap ring doesn’t flips down completely to “lock” the pin in place. Eventually the ring flips back up, pin falls out, and you find it under your tractor (actually most times, you never see it again).
I’d look around the tractor’s bucket, or more likely, the 3 point hitch connections and lift arms.
Don’t worry, when whatever it’s from falls apart , you’re going to know real quick where it goes.