This is the place that saves and stores SHSH blobs and APTickets from iOS 6. This was introduced when it was discovered that the Cydia SHSH Server had corrupted iOS 6.0 - 6.1.2 SHSH blobs and APTickets. A link to the TSS Center was once featured on the Cydia homepage, and can still be accessed using the List API:
SHSH only show up to (and including) iOS 6.1.6. SHSH blobs newer than this do not show up due to the List API needing updating for this.
TSS stands for Tatsu Signing Server.