Shrimad Bhagavad Gita chapter 4, verse 32 says that various religious acts / activities have been described by the God himself (through his mouth) in his speech (vani). That speech has been referred to as the Sukshm Ved. Further in chapter 4, verse 34, it says, that only a Supreme Saint will give you that supreme knowledge.
The essence is, that Supreme God Himself gives his secret. He does so by appearing on this earth and then gives his knowledge through his own mouth which is recorded as speeches (vani). The collection of that speech is the actual Sukshm Ved.
God Kabir appeared on this earth and gave the supreme knowledge via Kabir Vani (कबीर वाणी). This vani / knowledge is the Sukshm Ved or the 5th Ved.
Vedas and Gita have limited knowledge. Sukshm Ved has the way of worship of the Vedas and the Gita as well as the way of worship of complete salvation.