A symphony is a long piece of music played by an orchestra.
Symphonies are usually thought of as very beautiful and important music.
Examples:
The symphony was very long and had many parts.
I like to listen to a symphony when I go to sleep.
The symphony was played by a big group of musicians.
The symphony made me want to dance!
In the land of candy, the symphony is made of chocolate and plays itself.
The symphony was so good that I listened to it all day.