Can someone describe the purpose of OSX root level folders in laymen's terms?
It's a bit messy, and there is a degree of cross-overs, but the quick tour, without really going into the subfolders of these:

/etc, /tmp and /var are just symbolic links to subfolders in /private.

/etc generally contains configuration files.

/tmp is for temporary files used by running processes (also see /var/tmp)

/var is also used for somewhat temporary files used by running processes, but these tend to be less temporary and more often preserved between runs - e.g. logfiles which are usually in /var/log.

/usr contains binaries (executables) and developer-related files for core unix/BSD utilities. Things the user installs themselves should end up in /usr/local.

/bin also contains binaries but they tend to be more OS-fundamental than those in /usr - such as cp, cat, bash.

/sbin is as above but focused on routine administrative tasks such as mount and shutdown.  The s=system/superuser, usually you need root/superuser rights to use these commands. 

/.fseventsd is used by the OS to log file-system change events (such as folder creation) so that interested parties can be notified when the filesystem changes.

However, to run the risk of saying what you didn't want me to say - most of these are of little interest, and if you were to use any, you'd be most likely to be in /usr/local, /var/log or /tmp.

Additional information with a focus on the common Unix structure can be found on Wikipedia (Unix File System).