Okay little one, imagine you're playing cowboys and Indians. You know how cowboys have guns, right? Well, a Cossack is kinda like a cowboy, but from a place called Russia. But instead of a regular gun, they have a special kind called a musket.
A musket is a really old type of gun that shoots out small metal balls using a special powder called gunpowder. The Cossacks would load the musket with gunpowder and a little metal ball, and then aim and shoot at their enemies.
It was really important for the Cossacks to be good at using their muskets in battle. They had to be able to load it quickly and aim it well to protect themselves and their friends from the other side.
So that's what a Cossack with a musket is! They're like cowboy soldiers from Russia who use special guns called muskets to fight in battles.