A musket is a really old fashioned gun. It's a long, metal stick with a hole in it where you put gunpowder and a bullet. You have to hold it with two hands because it's very heavy.
To shoot a musket, you have to put some gunpowder in the hole, then put a bullet on top. You use a stick called a ramrod to push the bullet down into the hole. Then you point the musket at whatever you want to shoot and pull a trigger.
When you pull the trigger, it makes a spark which sets off the gunpowder inside the musket. The gunpowder burns very fast and explodes, which makes the bullet shoot out of the hole really fast. It's like when you blow up a balloon and let it go - the air rushes out really fast and makes the balloon fly around.
Musket guns were used a long time ago by soldiers in battles. But they were very hard to use and took a long time to reload, so they're not used anymore. Nowadays, we have much faster and easier guns to use.