A face smiling and wearing dark sunglasses that is used to denote a sense of cool. The nerd face emoji is a similar face, but with regular glasses.
On Snapchat, this emoji next to a contact denotes that you have best friends in common with this person.
Smiling Face With Sunglasses was approved as part of Unicode 6.0 in 2010 and added to Emoji 1.0 in 2015.
😎 Mutual Best Friends (Snapchat)