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.
We have two sizes, the PG-9 size is good for 0.158" - 0.252" diameter and this one, the PG-7 size, is good for cables 0.118" - 0.169" diameter. These two are the most common sizes for power & data cables, be sure to measure them so you have the proper fit.
Adafruit Best-Friend-Forever (BFF) Akiba over at Freaklabs.org has a great tutorial about how to use cable glands to keep waterproof enclosures waterproof, even with power or data or sensor cables coming out.