Okay, so you know how sometimes when you listen to a song, there's a person who is singing along with the main singer but their voice is not as loud? That person is called a backing vocalist, and they do a really important job.
Think of it like when you're playing a game with your friend. You are the main player, and your friend is your helper. Your friend is there to make sure you do well and to help you out if you need it. That's exactly what a backing vocalist does - they help out the main singer by adding extra layers of sound to the song and making it sound even better.
Sometimes the backing vocalist sings the same words as the main singer, but other times they sing different words to create harmonies. Harmonies are when two or more people sing different notes that sound good together. It's kind of like when you and your friend sing a song together, but your friend sings a different note than you do. It makes the song sound really cool, doesn't it?
So if you ever hear a song and think, "Wow, this sounds so full and amazing," chances are it's because there are some really talented backing vocalists helping out the main singer. They play a really important role in making a song sound great, even if they aren't always in the spotlight.