221B Baker Street is the address of the home of Sherlock Holmes.
When Madame Web took Spider-Man to 19th century England in another dimension to search for Mary Jane Watson, who had previously been lost in a time dilation portal, Spider-Man went to 221B Baker Street and asked Sherlock Holmes for his help to locate Mary Jane.
221B Baker Street first appeared in A Study in Scarlet when Holmes and Watson rented it from the landlady, Mrs. Hudson.
Watson moved away after his marriage to Mary Morstan following the events of The Sign of the Four. Watson had said that the monthly rent Holmes pays was almost equal to the full value of the house.