Okay, so you know how sometimes in music we have sad or happy feelings? Well, one thing that can make a song feel different is when we change the last note at the end of the song.
Usually, if a song is sad, the last note will be sad too. But sometimes, in a happy song, we might surprise you with a sad note at the end. This is called a "Picardy third."
Think of it like this: you're playing with your toys and your mom comes in and says it's time for bed. You might be sad because you want to keep playing, but your mom says it's time anyways. That's what a Picardy third does to a song – it's like a surprise ending that changes the mood of the song just a little bit.