Swag has several meanings.
Primarily swag meant loot or treasure that you aquired without payment or exchange. Usually items rather than hard currency that nevertheless held significant value. For instance pirates are often characterised as reffering to their ill-gotten-gains as swag. These days the word is often used to describe items given away during promotions or as free gifts.
In addition it is sometimes incorrectly used to describe a cocky or self assured attitude, usually but not exclusivley on the part of a young person.