T1, T2, T3,... denominate the different Tier of buildings.
Remember when starting? You only could build T1 buildings (workshop, stable,...). Once you bought land 7, you unlocked "better" buildings, the T2 ones (forges, beastiaries,...). And so on.
The difference between tiers is their relative strength (troop strength, leading to the visible stats) as well as their price. Plunder usually also slightly increases with increasing tiers.