It is Taiba, thereby meaning Medina. It drives away impurity just as fire removes the impurity of silver.
Sahih Muslim Book 15, Hadith 560
← Previous Hadith Muhammad, cancel my oath of allegiance. But Allah's Messenger (ﷺ) refused it. He again came and said: Cancel my oath of allegiance. But he (the Holy Prophet) refused it. He again came to him and said: Cancel my oath of allegiance, but he refused. The desert Arab, however, went away (cancelling the allegiance himself). Thereupon Allah's Messenger (ﷺ) said: Medina is like a furnace which drives away its impurity and purifies what is good.

Sahih Muslim Book 15, Hadith 559 Next Hadith → Allah named Medina as Tabba.

Sahih Muslim Book 15, Hadith 561