Single point slings got that name because they only have one attachment point on the rifle, generally towards the rear of the receiver or stock. Single point slings offer a lot of flexibility, making transitions from shoulder-to-shoulder a quick and effortless affair.
They also allow the rifle to sit directly in front of you (or under one arm), making transitioning from carry to ready very fast. When using a rifle-style pistol, or compact sub-gun style firearm with a PDW-style collapsible stock or a folding stock adapter, you can even push out against the sling and use the tension to improve control in an emergency.