Classic explain chromie time

okay so im confused. i started playing wow for the first time this week and i got my main character to level 60. my friend and i decided to make another character and to do another campaign together. to my understanding you have to complete the battle for azeroth to do the burning crusade but when looking at the list there is a portal to outland? what is this? when i look at expansions list its not on there and im just trying to figure out exactly what it is via /r/wow https://ift.tt/mOoEyaq