When someone is "nonchalant", it means that there's something big happening, but they're acting like everything is normal. Here are some examples of people acting "nonchalantly":

Your friend is winning a prestigious award, but he keeps saying that it's not important.

A soldier is standing on the street with a big gun, but has a bored look on his face.

A guy wants to ask a girl out and is really nervous about it, but he tries to start a normal, casual conversation with her.

Being "nonchalant" can be bad if you're in an important situation but it seems like you're not taking it seriously.