In other words, Tiantai teaches that the everyday, phenomenal world (the six realms of what we see around us, including also the gods, hungry ghosts, and so on) is not ultimately different from nirvana.
After we attain nirvana, then, the world is still the same place, but our understanding of it has changed with our higher consciousness.
The six realms of existence, and the four enlightened being realms (sravakas, pratyekabuddhas, bodhisattvas, and Buddhas) are collectively known as the ten dharmas. There is no level of existence outside of these ten realms.