T1 stands for Tier 1, it's just a guide used on this website to let people know how much to expect to buy/sell a villager for based on popularity. You can see the guide here: click
Hope this helped!
T1 villagers are villagers that are most popular/people look for those villagers the most. For example some T1 villagers are: Lolly, Marshal, Julian, etc. because of how they look/act/or how their room looks like