Sasuke stands firm and with his nonchalant appearance, he begins talking,
2T is second stage of sharingan, that is 1T sharingan evolves into 2T sharingan. As the name indicates, 2T sharingan eyes are red in color with 2 tomoes, but that is the most obvious thing about appearance.
Apart from change in the appearance of the eyes, this stage provides access to some important abilities, one of them the ability to see through the genjutsu, typically the higher ranked ones(I guess, till C-Rank). Also, it grants you the ability to track movements of your opponent more smoothly and copy them swiftly, rather this ability is more useful as compared to previous one. Since, you get to track the movements almost twice as fast as what you can track using 1T sharingan. One can use this tracking abilities to track the handseals and hence predict the upcoming ninjutsu and think a counter for it, in advance.
Another interesting point to note here is about copying the movements, user can copy the movements, confusing opponent, which is another important ability to have.
Sasuke then adds, one can keep 2T active as long as he has chakra, but obviously he needs to fuel it with chakra or he can simply disable it when it is not needed or when he is low on chakra.
OOC: Sharingan requires one move to active, in which user needs to post the description of the sharingan.