Explain questions expect you to give the reasons for something using your scientific knowledge. You should make sure that your answer flows sensibly, linking key ideas. You should not just give a list of reasons. If they wanted a list of reasons then that is what they would ask for.
Example: Explain why stars give out heat and light.
Stars contain vast amounts of hydrogen which is under high pressure and at a high temperature. This means that nuclear fusion can occur. This is when hydrogen nuclei join together to form helium nuclei. The process releases large amounts of energy as heat and light.