Bias first off means to favor one side better. Not all the source will be all facts. (Ex. Pericles is one of the most brilliant leaders of ancient grease) factullay based articles would say (ex. Pericles is one of the most well known leaders of ancient grease)
When the creator of a source is bias towards the subject they are writing about their source will most likely make the side that they are bias towards sound better than the other side and may leave out certain parts that may make their side sound. The source may also leave out critical information about the other side that you may need, so unbiased sources are the best to read.
