is this guy talking about the performance of green glue for soundproofing?
He's talking about "sound transmission class", often described as "STC". This is apparent from context, if you're familiar with sound reduction materials, but it's even made explicit earlier in the video. The host describes a variety of materials which are used to improve the sound blocking characteristics, and he uses the full name in those other sections.

That said, it would not be entirely accurate to say that the green glue caulking itself has an STC of 5. Nor is that what the host is really saying. The STC really applies to the entire barrier, after all the parts are assembled. So the caulk itself doesn't have a particular STC itself, but when used to join the two sheets of drywall, increases the STC of the assembled barrier by about 5 STC points, as compared to just doubling up the drywall.

See also a related measurement, "noise reduction coefficient". Useful references include:
http://www.acoustiguard.com/blog/414-stc-nrc-and-iic-what-s-the-difference.html
https://www.soundassured.com/blogs/blog/what-is-noise-reduction-coefficient-nrc
https://www.soundproofidea.com/difference-between-nrc-vs-stc.html