Hi lookaround,
Now I could be wrong but that looks like the drain valve for your boiler. At the moment to me it looks to be open. You'll know for sure if you fill the boiler and water gushes out the drain pipe outside
To close it just turn the blue cap on top 180 either clockwise or anti, I don't think it matters. Also, it looks like that drainage valve automatically opens should the temperature fall below a certain level. It's there to protect against freezing and bursting pipes should you forget to manually drain the boiler. If it does activate, the circular blue button at the bottom will pop out. When refilling the boiler again just push it back in. Try and see how it goes.
As you probably know it is important to drain the whole system of water as temperatures plummet. Blow out all taps etc too. Plenty of info on this site for you to research! Hope that helps.