Star San is an acid based sanitizer from Five Star. It is a no rinse, non toxic, odorless, tasteless and fast acting sanitizer.
When in action StarSan creates a foam which covers surfaces and increases contact time to ensure proper sanitization. The famous line "don't fear the foam" means that it is perfectly safe to rack beer onto/into without concern of it affecting any aspect of your brew.
1fl.oz. is enough to create 5 gallons of sanitizer. This equates to 1.6 mL per Liter.