Yeah, the dates are hard to read. I miss them unless I'm really looking for them. I just wondered because wblock hasn't posted here in a long time (to our loss) and FreeBSD-9x is pretty old by now.
svnlite checkout et al will pull the source code and put it on your machine in /usr/src. It is sometimes necessary to build various ports, and apparently, even to install some packages.
svnlite is the program, checkout is what the command will do--there are various things svnlite does, checking out, in this case, means pulling the latest source code.
svn.freebsd.org will go to one of the FreeBSD.org's main svn servers (that have this source code) the base/releng/12.0
is, if you're running 12.0, the version of the source code you want--the base, meaning the main system, releng, meaning a release, and 12.0, which release you want. (It might also be 11.2 if that's what you're running). /usr/src is where you're going to put this checked out source code.