In Australian historical terms, a swag is a portable shelter that is rolled (usually with belongings inside) and carried on a person's back.
Before motor transport was common, foot travel over long distances was essential to workers who were travelling in the Australian bush and who could not afford a horse. Itinerant workers who travelled from farm to farm shearing sheep in the late 19th and early 20th centuries were called "swagmen" because they carried all their possessions in a swag. This image was immortalised in Australian culture by the song Waltzing Matilda.
A modern swag is a waterproof canvas sleeping compartment that is sometimes bug-proof. All swags come with a foam mattress, and can comfortably be slept in with the addition of a pillow and sleeping bag. When rolled up the swag is lightweight and very compact, making it ideal for storage and transport. It is typically easy to erect and roll up can be done quickly.
There is also a Norwegian chain of stores named Swag, that focuses on extreme sport themed clothing.