I'd really appreciate it if someone could explain tag prefixes to me using specific examples.

I don't see much use for them, but I'm afraid I might be missing something.

The prefix is added to the title and that can be helpful for users and SEO. For example: a prefix could define the type of topic, e.g. “Question”, “Feature request”, “Bug report” or specify other details. For example, let’s say I have a forum about Adobe products. I could give users the choice to put “Photoshop”, “Illustrator”, “Premiere” and so on in the title, so users will instantly know which app the topic is about.  

All that being said: I don’t use prefixes on any of my sites. While there are these benefits, it requires users to use the system correctly and that is rarely the case. So it might do more harm than good. But that would depend on the community and the tags setup.