'On fleek' is a new bit of slang meaning 'perfect', or at the very least, 'awesome'.
The origins of 'on fleek' are murky, the earliest appearance being on Urban Dictionary in 2003, where is was simply defined as "smooth, nice, sweet"; this definition has since been removed, perhaps due to the flood of recent additions, spurred by the inexplicable internet fame of one Peaches Monroee, who used it in a fairly vapid and extremely popular Vine.
"We on dis beetch, for to get krunk; eyebrows on fleek, da fuck."
This Vine was of poor quality, limited content (the full text is shown above), and accompanied by an annoying beeping in the background, as she is apparently sitting in her car with the door open and keys in the ignition. She got 20 million plays, 492,000 likes, and 411,000 revines. It was shortly reposted to YouTube, where it has gotten another 2.5 million views. She has used 'on fleek' in later Vines, although these have not taken off to the same extent.
In short order Ariana Grande was seen (and Vined) performing the original Vine set to music on MTV (Aug. 17th, 2014), Taco Bell bragged "Taco Bell on fleek" on their official twitter feed (Oct. 17th), the International House of Pancakes Tweeted "Pancakes on fleek" (Oct. 21st), and Kim Kardashian Instagrammed herself with bleached eyebrows with the hashtag “#EyebrowsOnFleek” (Nov. 4th).
'On fleek' is probably going to become more common before it fades, with Google Trends showing it trending strongly upwards and Twitter flooded with #onfleek. It currently appears to be much more common in America, and is mostly used in the context of fashion, but this is surely a temporary state of affairs.