ss - slow speed
ff - fast speed
tt - typical speed
In your synthesis, you generate netlist from all three libraries. And provide the netlist for post-sim verification.
However, some people only use tt (typical speed) for synthesis because the netlist has no information from layout. ff and ss are only estimation. The slow (ss) and fast (ff) corners can be more accurate after layout is generated. And use the layout sdf file for corner speed verification.