What is report server temp db?

The ReportServerTemp DB stores temporary reporting service information. This includes both session and cache data.

Reporting services will not function properly without the reportServerTempDB  database. If the data base is lost you can simply rebuild it. 

 i will give some tables and related functions in Reporting Server Temp DB

Chunk Data Table: It will stores report definition and data for session cached reports and cached instances.

Execution Cache Table: It will stores execution information including time out for cached instances.

PersistedStream Table: It will stores the session level rendered output for an individual user.

Session Data Table: It persists individual user session level information, including report paths and time outs for given session information.

Session Lock Table: It will storesTemporary storage to handle locking of session data.