It's a traditional greeting that means "May peace and prosperity be upon you."
Here's a little more about it:
* Pronunciation: "Om" is pronounced like the English word "ohm" and "Swastiastu" is pronounced roughly as "swa-stee-as-too."
* Usage: It's a respectful greeting used in formal and informal settings.
* Response: The standard response is "Om Swastiastu."
You might also hear people say "Selamat pagi" (Good morning), "Selamat siang" (Good afternoon), or "Selamat sore" (Good evening). These are Indonesian greetings that are commonly used in Bali.