We get asked a lot of questions about ridgelines, so I'm going to go through this from top to bottom. We'll call this "Ridgelines for Dummies" because this is designed to cover the basics, explain what a ridgeline is and what functions it serves.
What is it?
A ridgeline is a strong bit of cord attached to the ends of the hammock, running parallel to the ground. By attaching to the ends, it forms a top to the hammock and appears as if your hammock is hanging from the ridgeline.
What does it do?
By attaching reasonably snugly to the ends of the hammock, it controls the hammock's "sag" which has a lot to do with how the hammock hangs, how comfortable it is, how flat it becomes and more. What the ridgeline provides is consistency.
What is it made of?
You want a ridgeline made of a low stretch cord that holds a lot of weight. In our opinion, something light is better. Avoid nylon (it stretches) and polyester cord is usually fat and heavy comparative to some other options. The most common recommendations are Lash It (TM), Zing It (TM) and Dynaglide (TM), though we've heard of some folks using cheap mason line that can be picked up at a hope improvement store. The diameter, strength and materials used in mason line vary greatly, so unless you know what you're getting will work, we'd recommend other options.
How does it work?
Our favorite is the fixed ridgeline at 83 percent of the hammock's length. This provides a deep, comfortable sag offers a nice, flat lay and has minimal shoulder squeeze. The easiest method is to tie a loop on each of end long cord. Our method of attaching a fixed ridgeline is outlined on our site; by opening the channel loops on the hammock, thread the RL loop over one the extended part of the channel loop and reaffix the larkshead knot.
A second option is an adjustable ridegline. This one has a fixed loop and an adjustable loop. You can take the fixed loop and attach it in the method described above, but you'll need a connector of some sort to attach the adjustable end. We don't recommend adjustable ridgelines for long-term use because they can lose their positioning and do not provide the consistency that is perfect for ridgelines (as it may move tighter or looser when you install it). But it's perfect for finding the right length for a fixed ridgeline. We use them to dial in the lay of a new hammock, then, when the sag feels just right, we measure it and make a fixed ridgeline in that length. Ridgeline biners are a simple connector on the adjustable end of an adjustable RL.
Tight or loose?
Getting your hammock between two trees exactly the same distance apart each time you hang is impossible and finding the perfect 30 degree hammock-suspension-to-tree angle is likewise difficult to get each and every time. What this means is that sometimes the RL will be tight. Sometimes it will be loose. If you use whoopie slings, you can adjust the hang of the hammock to get the ridgeline snug, but not too tight. You don't want your ridgeline to sag. But you don't want it to have to support too much of your weight either. Ideally, it will be pretty tight (like a loose bass guitar string) and will transmit the weight it holds to the suspension system. If it sags, try again. If it sounds like a high pitched guitar string, loosen it up a bit. Don't overthink it. If you use a good, high strength line (like 2.2 Zing It), you shouldn't sweat it too much. A good rule of thumb is that when getting in the hammock, you should be able to, with some effort, pinch a bit of ridgeline and get it to turn a little.
Reasons to use one?
We've discussed consistency of the hang. That's the key benefit, but folks find it also makes a great place for storage of items. Lights with clips attach to the RL, as do eyeglasses (I keep mine there). A ridgeline serves as a key support for the BIAS Buginator bug net, too. So consistency, storage, and attachment of accessories/personal items all seem to be pretty good reasons.