LOL, yes, there is a lot of terms you can learn, as you'll no doubt soon see.
Re: unicorn ... of course the original (pre-poly) meaning was a mythical/magical horse with a single horn on its forehead. In the poly community, you'll sometimes hear the word used in a humorous, ironic, or sarcastic way.
The idea is, suppose there is a couple (e.g. a husband and wife) who are looking for a single woman to date/form a household/family with. This type of situation happens fairly often, in fact most poly veterans will nod that they're familiar with the words, "We're a loving couple who is looking for a third to 'complete' us." This "third," this woman that this couple is seeking to add to their lives, is sometimes called a "unicorn," because in some people's minds, that couple is looking for "a mythical person, a person who doesn't exist."
It's just tricky if the couple has the expectation in mind that this "unicorn" person will be "hot," single, and dedicated solely to the couple. It's even worse if they find their unicorn, and then make that new woman take second place to their "original-couple" relationship and desires.
On the other hand, not all "unicorn seekers" are so inflexible or insensitive, and in truth, "unicorns" do exist (though people disagree about how common/rare they are). So using words like "unicorn," "unicorn seekers," and "unicorn hunters," can be derogatory, discriminatory, and sometimes even hurtful to the people the "unicorn" words are referring to. That's why I, personally, have tried to get out of the habit of using the word "unicorn" (though I admit that bad "couple-seeking-a-third" situations do exist, and are a problem).
Whether you use the word or not, it's probably useful to know what it means, and a little about its connotations, so you won't be lost if you hear someone else say the word in a conversation. So, this post has hopefully armed you with that information.
Anyway, I'm glad you're here, and would like to welcome you to our forum, Chocolate Barbie. I hope you'll find a bit of home in this far-flung internet place.
With sincere regards,
Love means never having to say, "Put down that meat cleaver!"