That is a very intelligent question and a clear cut source of confusion when ops and devs talk. It means different things depending on context, and especially so in a web hosting context.
root can as far as I know officially mean at least:
- the system full access privileges user.
- the file system base directory, a.k.a. /
- the web server shared file system base directory from which a web site is served, a.k.a. web-root.
- the first slash after the hostname in a web url address, a.k.a. the uri root.
- the shared script path common to several sites or uri:s within a site, a.k.a script path, root-link, web-root relative link, root-relative link.
The last two are usually not referred to in documentation and reference material written for sysadmins, a fact which adds relevance to the question.
I have heard each of these being referred to as 'the root' at some time or other, by some professional and highly skilled web developer or other. This has been eye opening as I read a lot and thought I had the 'root' term well defined. It is thus easy to misinterpret the real intention, due to differing terminology both between ops and devs but also between different subroles of devs (as is my impression).
I wouldn´t be surprised if there are a good number of other meanings, both valid and misconstrued, in addition to these.
So your task is figuring out what on earth your dev is referring to, which is not always easy :-)