Talse Uzer Story (T.U.S) is the overaching story that Tower of God is set in. It is a universe in which there are several central stories left as forever unchanged records. In other words, in T.U.S the past cannot change and the future is already set.
Tower of God is one of the "several stories" in T.U.S, but as these stories do not have much connection to each other, there is no reason to understand T.U.S. You can enjoy Tower of God even if you ignore T.U.S entirely.
For now, Tower of God is the main story since SIU hasn't published any other T.U.S .