So, you know how there are different notes in music like Do, Re, Mi, Fa, So, La, Ti, Do? Well, sometimes these notes can be made to sound a little different by adding little symbols next to them. One of these symbols is called a "flat."
When you see a flat symbol next to a note, it means you are supposed to play or sing that note a little lower than it normally sounds. It's like when you try to sing a high note while holding your nose - it comes out lower.
For example, if you see a flat symbol next to the note "Fa," it means you play or sing it like "Fah" but a little lower, almost like "Fahhhh."
Overall, flats can make music sound a little bit different and add some variety to the notes we hear.