That means Vanguard VTI tracks the stocks that are contained in the CRSP US Total Market Index. So, if CRSP adds a new holding to its US Total Market Index, Vanguard VTI is legally bound by its prospectus to add that stock to VTI and to do so in a manner that assures VTI returns track the CRSP US Total Market Index returns as closely as possible (meaning they own every stock CRSP does and in the same proportion that CRSP does).
This is not just a 'good idea'. It is legally binding upon Vanguard by virtue of the VTI prospectus.