It's a very common and informal greeting, used between friends and family.
Here are some other ways to say hello in Polish, with different levels of formality:
* Dzień dobry (pronounced "dzen doh-bree") - Good day (formal)
* Witam (pronounced "vee-tam") - Greetings (formal)
* Cześć (pronounced "chest") - Hi (informal)
* Siema (pronounced "see-ma") - Hey (very informal)
* Witaj (pronounced "vee-tie") - Welcome (formal)