It is an abbreviation for "optional." Historically, /opt has been used to install non-traditional or non-standard software that isn't considered part of the main underlying system. In the GNU/Linux of today, I find a lot of software that aren't installed using the distribution's package manager will install themselves into /opt in order to avoid conflicting with the package manager which typically installs files under /usr. That being said, it doesn't really matter that much as long as you're careful. In today's landscape, /opt is just another directory that you'll sometimes put things, but those things can also be put elsewhere.
If you're interested in discussing more, hit me up!