A wagner tuba is a special kind of musical instrument that looks kind of like a tuba, but it's smaller and has a different shape. It's also a brass instrument, which means you blow air through it to make music.
The wagner tuba was invented by a man named Richard Wagner, who was a really famous composer of music for operas. He wanted a new instrument that would make a certain kind of sound that he heard in his head when he was writing music. So he worked with some people who made musical instruments to create the wagner tuba.
When you play the wagner tuba, you can make different notes by pressing on the buttons or keys with your fingers. The sound it makes is kind of like a cross between a French horn and a tuba. It's often used in big orchestral pieces, especially in operas.
So if you ever hear some music that sounds kind of like a tuba, but not quite, it might be a wagner tuba. And now you know what it is!