It's a 1" male to male plastic pipe connector with a valve - strangely I can't find anything equivalent despite ages searching online. I've got a 1" to 1" male to male connector without a valve but could really do with a replacement for this one.
(It goes on my waterbutt which has a 1" female connector & I use it to attach a female threaded -to-hozelock connector from which I can then take off a stub of pipe for a watering can or a longer hose - but easiest to have the valve right on the butt)