괜찮아 is a very common phrase that can be used for multiple purposes. It is the conjugated form of the verb 괜찮다, which means to be fine/okay/average state.
In addition to being used to say “that’s okay” or “no problem,” 괜찮아 can also be used to say “I’m okay” or “I’m fine (I’m not hurt).”
You can also use it as a question:
Are you okay?/Is it okay (if I do this)?
Need a refresher on verb conjugation? Go to my lesson on conjugation.