To be impious is to be disrespectful of god or duty. When someone is being impious they are doing things that their church, synagogue, temple, mosque, school principal, government or parents would find unacceptable.
When you don’t show reverence for religion or god, you are impious. The adjective impious is related to the word piety, which means religious reverence. To be impious is to be without piety. Being impious is similar to being blasphemous, but it’s a little more passive to be impious, while blasphemy is more actively insulting. Also, when you act out against tradition or dutifulness, you could be considered impious. If you dodge a military draft, you will likely be considered impious.