Help me identify this part please

I decided to attach a hand shower bidet to my toilet. So I unhooked the supply line from the tank. As I was getting everything set up, a little plastic doohickey fell on the floor. I think it fell out of the hole in the tank where the supply line had been connected. It's clearly not a new part, so I'm sure it isn't part of the hand shower I'm installing.

It is 3 cm long and 1.8 cm in diameter.

I've attached a photo. Could someone who knows more than I do about toilets tell me what it is, what it does, etc.?

Thanks!
David