It stands for Stuff We All Get. It is Mendix stuff that you would like to have such as T-shirts and flip-flops etc. In the Mendix community you have a lot of people who are proud to be Mendix developers and we would like to show that with our Mendix SWAG. You can score points in the Mendix community by doing assignments (see your profile page) you can then order the swag for FREE with those points.
check out this link: https://mendixshop.mendixcloud.com/index3.html
Cool right?
Not to be confused with SWAGGER ;) more info about Swagger here: https://docs.mendix.com/refguide/open-api