Bothsidesism occurs when the media presents an issue as being more both-sided, contested or balanced than it really is. For example, there is overwhelming evidence that climate change is real, even though some (very few) scientists disagree. By giving equal voice to scientists on both sides, the media creates an illusion that the issue is contested or a matter of perspective, even though it isn't.
Similarly, some media outlets may blow the minority opinion out of proportion, making it seem like there are two equally strong sides, even though this isn't the case.
Funnily, bothsidesism is a bias that occurs when the media tries to minimize bias (by presenting both sides of the argument). The average media consumer may feel more informed (since the issue seems more nuanced and complicated) but actually they are misinformed.
We are often told that it is good practice to include both sides of the argument and consider all perspectives. However, in some issues, it's better to not give equal weight to both sides or to "come to a compromise". One can consider all arguments, without giving equal importance or merit to each.