When Obito told Sasuke the truth about Itachi after their fight he mentioned that Itachi had a girlfriend/lover. Since that’s all the information we had at that time, everyone was wondering who she was. All we knew is that she was an Uchiha and she was killed by him during the massacre.
A lot of information about her is revealed in the 2 canon novels about Itachi’s past, Itachi Shinden: Book of Bright Light and Itachi Shinden: Book of Dark Night (they are currently adapted to some extent in Naruto Shippuden). The events in the anime will not be mentioned since they did not stick to the novels and it is considered filler.
Izumi Uchiha was a kunoichi from Konohagakure and a member of the Uchiha Clan. She was a friend of Itachi Uchiha since their childhood, and was infatuated with him.