A tandem axle means that there are two axles on a single travel trailer, one in front of the other.
When there is only one axle on a travel trailer, it is known as a single axle. Heard the Bambi name before? If you look back through the history of Airstream, Bambi was a nickname for our smallest single-axle travel trailers. But now Bambi is its own official family of lightweight, space-maximizing options ready to deliver a huge step up from the tent camping experience. The Caravel is also a historical single axle travel trailer name that has been revitalized with a modern layout and features.
Learn more about the differences between a single axle and tandem axle travel trailer-
https://www.airstream.com/single-axle-vs-double-axle/
or
https://www.airstream.com/blog/single-axle-or-double-axle-travel-trailer/