The definition of a reliable source is any source that has competence in the field of interest, without any biases or conflicts of interest related to the topic. Finding reliable sources is important in order to ensure that what is written or reported is accurate.
Reliable sources are needed for many different tasks and assignments. High-quality journalism involves only relying on sources that can provide accurate and trustworthy information. Writing a technical paper or study requires studying and citing other papers or studies, but if unreliable sources are used, the credibility of the piece could be called into question.