You use "going to ___" to talk about what will happen in the future. "Going to ___" is more natural than "will" for most situations where you're expressing someone's plans to do something:
They're going to meet us at the airport.
I'm just going to stay home and relax this weekend.
English learners often over-use "will" when "going to" would be more appropriate. In general, when you want to talk about something that you've already decided to do, use "going to":
If you've just decided something and want to announce your decision, use "will":