The Public Test Server (or PTS for short) is a server that the devs use to test new content. Inside the PTS you will be able to test features coming in the next patch as well as being able to buy some items in the shop for free. Progress made in the PTS will not carry over to Live.
Though all players have unlimited funds on the server, not all items in the store are able to be purchased. For example, Chaos Chests are not allowed to be purchased as they may allow players to see what loot is in the chests for that week. Most packs are also blocked; however, Dragon Coins, Dragon Eggs, Mounts, Costumes, Wings, Flasks and Emblems and a few other items are all available. Some newly implemented packs that are not yet available on Live can also be purchased before the official release.
This video will tell you how to access it.