The "Kotti" is the short form for "Kotbusser Tor" which is principally a traffic circle, cut through by a U-Bahn viaduct.
The term is also used for the whole area with the surrounding houses, bars, and shops.
Opinions about the Kotti are diverged. For the mainstream media this is a poor area with a high crime rate which makes it now and then into the headlines. For others it is a exciting place where many different cultures live together, forming a culture of its own.