That link is an article about how to install Xcode command line tools*, which you often need to build packages from source in R. There's another nice description in the What They Forgot to Teach You About R book (online):
Summary point of that section being that the minimalist approach to take is to run:
xcode-select --install
from the shell.
* That's actually a really helpful error message, most of the time you just have to figure it out by seeing that xcrun is somewhere in your installation errors (e.g. see here).