A sling is a type of strap or harness system that allows you to carry a gun on your body. There are different styles of slings each with their own pros and cons that go beyond being able to carry your gun hands free.
Two-point slings - this is a common type and most traditional old school sling. Two-points means it mounts at two locations. This type of sling functions as a simple strap which you would use by slinging the rifle over your shoulder/back. Certain makers have improved upon this design with their own features to add features like the ability to transition to different lengths for improved shooting stability and so on.
Three-point slings - not seen as much these days but still another type of sling that offers unique advantages. This type is constructed more like a harness system and the three points where the sling is mounted is the front and rear of the weapon, and your body. Retention is really good and the gun is still accessible at the front of the body and hanging off while your hands are free to perform other tasks. Transitioning to the gun is quick because the gun is right in front of you hanging down your torso. There is a lot of material though so with most designs you can easily transition from strong side to weak side if need be.
Single-point slings - these types of slings mount at a single point, usually at the back of the firearm with a loop of strap going over your shoulder. This is the simplest of the slings and meant for short term usage where retention is somewhat important but not crucial to where it needs to be firmly secured.
Q: What sling is right for me?
A: If you see yourself playing very short games or in something very fast paced, a single-point sling is plenty adequate for the task. You can still lower your weapon and have it dangle as you switch to a secondary gun like your pistol or require both hands to perform a task.
For a longer term type of carry a two or three point sling would probably be more comfortable and make more sense.
Q: How do I mount a sling to my rifle?
A: First you have to identify if you have any mounting points for a sling on your rifle. If you have none or the ones you have are not compatible with the slings you are looking at, then you might have to get adapters or add-ons to your gun that will provide you with a mounting solution. Slings can be mounted several ways: by running the strap through the loops on the gun, quick detach loops, and mash hooks.
Q: How do QD (quick detach) sling loops work?
A: These have buttons with springs and detents built in and a standard sized socket that allows this type of loop to be universally compatible with the many newer guns that are equipped with QD sockets from the factory. The sling will usually be installed to the push button QD swivel.