According to the Cochrane Collaboration, reporting biases arise when the dissemination of information is skewed by the “nature and direction” of the study results. Reporting biases mean that studies with positive results are more likely to be published in the medical literature (publication bias), are published more quickly (time lag bias), are more likely to be published more than once (multiple publication bias), are more likely to be repeatedly referenced by other researchers (citation bias), are more likely to be published in English (language bias), more likely to be published in peer-reviewed journals with high indexing standards that make it easier to find the study (location bias) than studies which find that the drug or treatment they are testing has no effect compared to placebo.
Taken together, reporting biases mean that published studies and reviews of published studies may overestimate the effects of a drug or treatment. For more on the causes and consequences of reporting biases, see this 2010 study in the journal Trials.