If you just asked yourself, "Get what? What is the 'It' that I'm supposed to get?", that's a really good sign that you don't "get it".
Almost all of my friends "get it". There are a few who don't, but all of my really close friends "get it". It doesn't mean I don't like people who don't "get it". It just means that they are inferior.
If you were insulted by that last comment, clearly you don't "get it". I'm being sarcastic... sort of.
One of my friends who REALLY "gets it" is Christine. It's not something she learned, as you can't learn how to "get it" from books, or school, or even life experience. She just has an extremely high "getting it" aptitude.
Here's a perfect example. Christine and I understand that drawing hats on pictures of birds is hilarious. Therefore, we can send the following to one another...
(Christine's)
(Mine)
... and we both get it. This is highly amusing to the two of us. If you're laughing at this, you're set.
Another one of my friends with a high "getting it" aptitude is Ricky. Just yesterday he sent me the following link: http://theendofhumor.blogspot.com/.
If you're not laughing at this point, I'm not quite sure why you're still reading.
Here's an easy test for if you get it or not: