The term is an acronym for the phrase "smiling at screen". This is to replace the commonly used acronym "lol" (laugh out loud).
This is used more in the business setting when talking via AIM or any other online chatting service. You in no instance will ever truly be laughing out loud while at work; you will chuckle at best.
kevin: hey did you read that email chuck sent?
jay: yea i got it while i was in a meeting. made me sas.