When you say that something "is a big deal", it means that it's important. But when you "make a big deal out of ___", it means that something is not that important but you're acting like it is. You can use this phrase is situations such as:

• When a family member is planning a big party for your birthday, even though you don't care about it.
• When someone is getting angry about something that you think is just a small mistake.
• When you lend someone some money and they thank you too much.

The phrase "make a big deal out of something" is used in casual speech.