A brokerage session exists between an authenticated/logged-in client platform (such as Trader Workstation, or your Web API client application) and IBKR's backend trading infrastructure. Authentication relies on an IBKR username, and once authenticated, that username is said to have established a brokerage session on that platform in use.
Only one brokerage session maybe exist for a given username at any time, across all platforms. Attempting to create a new, competing brokerage session may terminate a previous session.