Hi! Villager trading is basically how it sounds, where one villager is traded for another or it could be a villager for TBT (currency on this forum) or bells in-game. To obtain the villager you would like, you add the person that is offering him once you buy/win/take them, etc. and then you go to their house in that person's village and talk to the villager to convince them to move to your town, IF you have space that is.
Let me know if you have anymore questions! PM me or post on my wall, like I just did on yours! I saw you had a question on the thread, so I decided to try and answer~