These plastic quick connects from Melnor are the go-betweens for the hose and whatever nozzle, sprinklers or other hose-end attachments you may have. They’re especially good for quickly moving and attaching hoses from one faucet to another. I installed them on ALL my faucets (5) and hoses (perhaps 7) and external attachments (probably 10). I have used them for about a year and wonder how I ever got along without them. It takes less than a second (maybe 1/2 second) to attach or detach any hose or attachment. They are installed in pairs, a male and corresponding female connector, with the appropriate threaded fitting to attach to the faucet, hose or nozzle attachment, one on each side of the connection. You just firmly push the connector into its counterpart, and it easily pops into place — firmly means it does need a little pressure, but even a small child could do it. To disconnect, you push the green collar about an eighth of an inch in the one direction it’s capable of moving, and it pops off. (Similar devices have been in use in industry for a long time — on compressed air lines, for example). No more screwing and unscrewing (no more scraped knuckles); no more leaks from incompletely tightened hoses; no more stuck connections because some gorilla (i.e. me) tried to stop a leak by tightening too hard.
One type is designed so that when you disconnect from it, an internal plug pops into place and stops water from coming out. The other type, for between a faucet and hose, does not have the shutoff. When you disconnect the hose from the faucet, water will still flow and the faucet can still be used. There are other brands and styles; some are even made of pricier brass, but I recommend you stick with one manufacturer because connectors are generally not interchangeable between brands. And these inexpensive plastic ones from Melnor are well made: I have (intentionally) very high water pressure (> 100 psi, sufficient to burst hoses) on my garden faucets, and I have had no leaks from these connectors.