TREX is a search engine that used in SAP NetWeaver. This search engine primarily used in SAP Enterprise Portal, such as for Talent Management Specialist to find employee. TREX server recommended to be installed in separate machine, since during indexing process can consume high resources.
TREX need to be connected with the SAP back-end.
More information regarding TREX admin can be found here.
While to setting connection of TREX can be found on screenshots below: