Pretentiousness is acting like you know more than you really do, or like you're more important than you really are. If your friends complain about your pretentiousness, you might want to tone down the bragging a little bit.
It's no surprise that pretentiousness shares a root with the word pretend — pretentiousness is all about pretending to be incredibly important or supremely talented. The pretentiousness of the literary novel you've been trying to read might lead you to reach for your favorite Harry Potter book. And the pretentiousness of your friend loudly demanding the best table at a fancy restaurant might make you wish you'd gone to a diner by yourself instead.