When the word "myth" is mentioned, people seem to think immediately of the ancient Greek or Roman myths, or of the Egyptian myths. They think about the myths involving the false gods that the early settlers of these nations clung to. The common thing among all of these myths? They are all false.
In Paul's first epistle to Timothy, Paul warns Timothy against paying attention to myths. They do not point us to the one true God. On an interesting note, in Paul's second epistle to Timothy, Paul reminds Timothy of the "God-breathed" nature of Scripture (2 Timothy 3:16-17). This sharp contrast between the two shows us how God's Word will point us to the one true God, whereas myths will not.
Paul calls the myths to which he is referring both "irreverent" and "silly". Each of these assessments are accurate. Myths are irreverent because they do not give glory to the one true God. They are silly because it is silly to follow them.