How to use the Preview app on my iMac

An Apple technician just walked me through opening a stubborn image on my desktop using an app that's in my dock. It's called "Preview."

I suppose "Preview"has always been my computer, but I don't recall using it during the 15 or so years I've owned a Mac. Anyway, I would appreciate a brief explanation as to what the app "Preview" is used for.

[Re-Titled by Moderator]