TS!UNDERSWAP is a fan-made Undertale AU, or alternative universe, in which the roles of various characters are swapped. For example, Papyrus is the one who is determined to free the monsters from the Underground, while Sans is the one who is trying to stop him.
The gameplay of TS!UNDERSWAP is similar to that of Undertale. You explore the Underground, talking to characters and solving puzzles. However, there are some key differences, such as the fact that you can now fight Papyrus instead of Sans.
The "TS" in TS!UNDERSWAP stands for "Team Switch". This is the name of the team of developers who created the AU.
Yes, TS!Underswap Papyrus smokes. He is often seen with a cigarette in his mouth.
In Undertale, Gaster is a mysterious character who is said to have created the Underground. In TS!Underswap, Gaster is still a mysterious character, but he is much more sinister. He is the one who is responsible for the swap of the characters' roles.
In Undertale, Asgore is the king of the monsters. He is a kind and gentle ruler who is determined to free the monsters from the Underground. In TS!Underswap, Asgore is a cruel and tyrannical ruler who is determined to keep the monsters trapped in the Underground.
I hope this answers all your questions about TS!UNDERSWAP.