CDLC, which stands for ‘custom downloadable content’, means, in the context of Rocksmith, songs which have been coded by the Rocksmith user community in such a fashion as to be useable with Rocksmith 2014.
Disclaimer: Note that CDLC are limited to that version of Rocksmith; they are not portable to Rocksmith+, which is designed around an entirely different model; indeed, given that model, it is not possible at all to produce CDLC for that platform.
It is also not possible to make CDLC work with pirate copies of the game, so no support can be provided for that.
The information required to set up a Rocksmith system for CDLC is available on the Customs Forge site at this address.
Support in the event of experiencing difficulties in setting up CDLC is available on Customs Forge here, on the Customs Forge Discord discussion (the more experienced experts are typically to be found there), or by browsing the support page on this site, where the most frequent issues and their solutions are documented.