Fake news is when an ultra-liberal, ultra-conservative, or click bait group creates a fake or drastically misinterpreted story in order to create a frenzy or to drive online traffic. These techniques, usually propagated online, are deliberately designed to polarize and further push us away from being able to understand perspectives different from our own. If it sounds absolutely outrageous to you, it was likely designed to do just that. They are designed to get an emotional response from you so that you will get angry and share it with your friends in disgust.
Instead, do your research on every piece of news to make sure that it can be verified by high quality professional journalistic sources (see graphic of news sources above) or to be sure that what is being stated matches a primary source. Research can be done quickly in two ways: 1) URL - if its odd or if it is a slight change from a name you trust, it is likely fake. 2) verify it by running a search on it to see if it is covered in a legitimate and professional news outlet - if not, likely fake. Snopes has an article on spotting fake news as does NPR. Do your research before you believe or share news.