This is a Mexican drink made with lemon, chia seeds, and a sweetener (honey, agave syrup, sugar, etc.) It's been made for centuries in the highlands of Mexico.
Here's a photo of a jar of Chia Fresca, submitted by my colleague, Dr. Joseph Cahill. He photographed it in the market at Huehuetenango, Guatemala. 
My recipe for E-Z Chia Fresca:
To 1 glass of commercial lemonade, add a teaspoon of chia seed. Shake or stir to separate the seeds. Let sit 10 minutes until the seeds swell.
(Note that it's MUCH better if you use homemade lemonade!)
ALSO, please see Rachel Laudan's post for more information. She calls is "Agua de Chia" but it's the same thing.