In the context of the exam questions this means that: you should make something clear, or state the reasons for something happening. 

When this command word is used in an exam question you need to give scientific reasons for what you say. Making sure you use the “because” will help to guide your answer. The red blood cell contains haemoglobin BECAUSE this respiratory pigment absorbs lots of oxygen at the lungs and releases it to the tissues when needed. If there are lots of marks for the question you may need to explain other aspects of the process, such as how the diffusion gradients for oxygen and carbon dioxide change at the lungs and the body tissues.

This command is often used where the question is asking you to apply your knowledge and understanding to unfamiliar contexts.  It can be tricky to explain something you have not studied. Don’t panic! you are only being asked to apply what you do know. 

For example, a question might asks you to explain why Tarantula spiders have a blue respiratory pigment in their blood cells. You may know very little about these spiders and nothing about the colour of their blood. Don’t panic!, you do know why human red blood cells are red and the purpose of these cells. All you need to do is explain about the red blood cells you have studied and the function of the red pigment they contain. You are just applying what you already know and making an educated guess about the context, but make sure to refer to the details in the question, it is spiders you are writing about not humans and the colour of the cells is blue not red.