Imagine you have a toy tube that you can make noise with by blowing into it. Now imagine that toy tube is really big and can make really low and deep noises. That's kind of like a helicon instrument!
A helicon is basically a very tall and large tube-shaped instrument that musicians play by blowing air into it with their mouths. The sound travels through the long tube and comes out of a big bell-shaped opening at the end. Imagine it like a really long straw where you blow air in one end and it comes out the other end!
The helicon is a type of brass instrument, like a trumpet or trombone, but it's much bigger and more difficult to carry around. Some helicons are so big that they need a special stand to hold them up while they are being played!
Helicons are often used in big marching bands because they have a very loud and deep sound that can be heard from far away. They can also be found in some orchestras and even in some rock bands!
Overall, a helicon is just a really big, tube-shaped instrument that makes loud, low-pitched sounds when air is blown into it.