/private has been around for a loooooong time. Every Mac has it.
It is part of your Mac: the directory stores essential system files and caches, along with other databases that your system requires to operate.
I'll give a quick explanation contents of the folders /private/etc, /private/tmp, and /private/var, usually found in /private:
Data files for system use, configurations for various command line tools etc
System logs, mostly
More logs, databases for daemons, virtual memory swap file(s), sleep image etc.
In essence, deleting anything in this directory is probably not a good idea, save for a few files. If you do delete them however, some required essential files will get regenerated on startup.
It's probably a good idea to Google the exact full directory path of any system folders you intend to tinker with before actually beginning to experiment with stuff inside.
I hope this helps.