A "demo unit" refers to a product or device that is used for demonstration purposes. It is typically a sample or a version of the product that is made available to show how it works, without being sold to customers. Full definition
But you should be able to try out a live demo unit in a store and see if the browser works with the site you use. (woot.com)
I tried out a 3DS demo unit a while back, though, and the things just don't feel «right» to me in terms of responsiveness and degree of control. (nintendoworldreport.com)
They often send demo units and beta versions of new software for their partners and vendors to beta test and submit feedback. (goodereader.com)