Have you ever heard someone talk with a really funny sounding voice where they kind of make a "ng" sound at the same time they talk? That's what a nasal vowel is!
So you know how when you say some letters, like "ah" or "oh", the sound comes from your throat? Well, when you say a nasal vowel, like the sound in the French word "bon" or the Portuguese word "mãe", the sound comes from your nose instead of your throat!
It's like you're talking with your mouth open and your nose closed at the same time. That's what makes it sound kind of different and funny!
Some people might even think that it sounds like you have a cold or like you're talking with a stuffed up nose, but really you're just making a different sound with your mouth and nose together.
So now you know what a nasal vowel is - it's just a funny and different sounding sound that comes from your nose instead of your throat!