One-sided means only showing a single side of a complicated issue, or being biased. A newspaper article is one-sided if it presents just one opinion about a controversial topic.
When television news gives a one-sided version of the facts, it's impossible for a viewer to get a clear picture of what's going on, especially when most political subjects have so many differing opinions. Another, much less controversial meaning of the adjective one-sided is simply "having only one side." You might be pleased to realize that your math test paper is one-sided, with questions only on the front.