On fleek means "on point". This slang is used to refer to anything that's perfect or close to perfection. For example, if someone just got their hair done, they might say "My hair on fleek." If a girl with a beautiful derriere pass by, onlookers might say "her ass on fleek."
This slang is believed to have been started by Chicago teen Kayla Newman. Newman posted a video on vine after getting her eyebrows done, in the video, she says "“We in this bitch. Finna get crunk. Eyebrows on fleek. Da fuq.” Newman's vine has been looped over 30 million times since she released it in 2014.
This slang became really popular in the Hip Hop community after numerous rappers started using it.