The Brief: On fleek means on point or perfect.

DEEP DIVE
The term originated from a Vine made in 2014 by then sixteen-year-old Kayla Lewis. She initially used it to describe her eyebrows, but it now can be used to describe anything.
https://twitter.com/anjellarobles/status/1037646511645978624
Eyebrows on fleek. Dafuq? pic.twitter.com/iEmxOsXZwL
— luchh🇳🇬 (@luchymoe) September 5, 2018