A database that contains all of S.H.I.E.L.D.'s historical and administrative data. The actual hardware for the chamber is in the archives at S.H.I.E.L.D. HQ, the Triskelion, but it can be accessed wirelessly.
In "T.A.H.I.T.I.", Fitz and Simmons access the Echo Chamber to learn where Coulson was taken after he died in order to help Skye because the Echo Chamber keeps a record of all S.H.I.E.L.D. activities including travel arrangements. There was one file that stuck out as it was a record of a departure from the Triskelion, the day Coulson died, and the last person to access the file was Director Nick Fury. The file was encrypted, but it led them to the Guest House.