It means "thank you" and is a common way to express gratitude in the Persian language.
You can use it on its own or with other words to create a more elaborate expression of appreciation. For example:
* سپاس فراوان: (Sepas faravan) - Thank you very much
* سپاس از شما: (Sepas az shoma) - Thank you
* سپاس بابت کمک شما: (Sepas babat komak-e shoma) - Thank you for your help
* سپاسگزارم: (Sepasgozaram) - I am grateful
Remember that in Farsi, the words are written from right to left.