Imagine if a person has a barrel of very expensive wine, but he puts a dirty spigot on the barrel. The wine will taste disgusting when it comes out because of the dirt on the spigot. A person’s mouth is like a spigot. If a person davens from the heart with beautiful concentration, but his mouth speaks filthy things, his prayers become dirty. A person should always be careful of what comes out of his mouth, so he does not “dirty” his prayers.