We have some great waterproof and weather-proof items in the adafruit shop, but once you have a project built, you'll want to enclose it. But how to keep an enclosure weather/waterproof while still attaching cables for power, data or sensors? That's where cable glands come in. By installing these over the cable, they form-fit to make a seal, keeping your project box innards safe from the outdoors.
This particular gland is specifically designed for RJ-45 cables, commonly known as Ethernet or Cat-5 cables. This gland is meant to work with any existing Ethernet cable and works in two parts to allow attachment/detachment. First, install the dual-socket part into the enclosure (that's the part all the way to the left). It has two RJ-45 sockets. Next, take apart the gland and slide the Ethernet cable thru it, with the over-nut first, then the rubber gasket, then finally the contracting gland. Plug in the cable, and screw the first over-nut to attach it to the socket, then tighten the gland to complete the seal.
Please note, if you have a Ethernet cable with a over-molded boot, the boot can get in the way of the cable. you can cut the boot off (carefully!) or use non-boot-having cables.