Rep is short for reputation. (That's probably something you already know, but I never know when words go out of style, so telling just in case you didn't.)
It really isn't a popularity contest, but if you see someone who has participated for quite a while, and has none, or very few, it does give a moment to pause. It kind of hints that the person hasn't said anything worthwhile. It hints. Not to say it is true. But you can get enough likes that you do get another little green bar under your name. And likes are given as an "I liked that" or "I read that."
Reps are usually given as an "I really liked that." And when you get one, most people leave a comment, (that you can find), telling why. They earn you more than a single like, but how much more depends on who gives it to you. Somehow another it's based on how many reps that person has. The more reps they have, the more points you get toward another green bar.
Sometimes, jerks give reps to blast you privately, because, as you know, we don't have to leave our names when we do rep.
Does any of this earn you any special goodies from CC? No. It's more like having someone compliment you in real life. It matters strictly on what you think of the complimenter. If the compliment is hollow, it doesn't mean all that much. (Then again, anyone giving us a compliment on a bad day still tends to make us feel better.) If it's from someone you respect and know to only give compliments when it's truly meant, it is a good feeling.
But since you're relatively new and already have five reps after 100 posts, it tells me you are actively and positively participating on this site, therefore, not someone to brush off easily.That is how others might see it too.