Even though the term's been around for a while, I've only recently started to hear the word 'basic' used to describe my female peers. Apparently, this is short for 'basic bitch,' which, according to an urban dictionary entry from 2009, means 'a bum ass woman who think she the shit but really ain't.' So. It's derogatory. Also specifically directed towards women (although I've heard it can be applied to gay men, as well).
Just in case you're worried, here's a rigourously scientific test to find out whether you fit the definition of 'basic.' I checked 8 boxes out of the 119 qualifiers for being 'basic' -- I love bagels -- I do I do, yoga's changed my life, and what person doesn't own jean shorts? I don't particularly like Taylor Swift's music, but guess what? I have three sisters who do, three sisters who also love brunch, Instagram, have wedding boards on Pinterest, and -- that's right -- also own jean shorts.