The assignment is to write an essay explaining a concept. My teacher is pretty unclear what she means. Any ideas?

This type of essay is also called an expository essay:

http://owl.english.purdue.edu/owl/resource/685/02/

A concept is an idea. Yes, it's that vague! So it sounds as if you can choose just about any concept/idea you want and then write a paper to explain what it's all about. Answer these questions in your paper: why, who, how. Answer other questions, if needed.

Here are some possible topics:
http://712educators.about.com/od/essaysparagraphspapers/a/academic_expos.htm

When your teacher asks you to write an essay explaining a concept, she is likely asking you to thoroughly define and clarify a specific topic or idea. Here are some steps you can follow to approach this assignment successfully:

1. Choose a concept: Start by selecting a concept that interests you or one that aligns with the subject you are studying. Make sure the concept you choose is broad enough to provide sufficient material for an essay but specific enough to be effectively explained within the given word count.

2. Conduct research: Gather information about the concept from reliable sources such as books, academic journals, or reputable websites. Look for definitions, explanations, examples, and any relevant theories or perspectives related to the concept. Take notes and make sure to properly reference your sources.

3. Understand the concept: Before you start writing, make sure that you have a clear understanding of the concept you have chosen. Carefully read and analyze the information you have collected, paying attention to any key elements or underlying principles that help explain the concept.

4. Outline your essay: Create a clear and logical outline for your essay. Consider dividing it into sections such as an introduction, background information, key components, examples, contrasting viewpoints, and a conclusion. This will help you organize your thoughts and ensure a coherent structure for your essay.

5. Introduce the concept and provide context: In the introduction, provide a concise definition of the concept and explain why it is important or interesting to discuss. Give some background information to familiarize your readers with the concept and its significance in the field or subject area.

6. Explain the main components: Use the body paragraphs of your essay to explain the main components or aspects of the concept in detail. Define and explore each component separately, providing clear explanations, examples, and supporting evidence. Make sure to avoid overly technical language and use a balance of academic sources and accessible examples to help your readers understand the concept.

7. Discuss contrasting viewpoints (if applicable): If there are different theories, interpretations, or perspectives related to the concept, present them in a fair and balanced manner. Compare and contrast these viewpoints, highlighting their similarities and differences. This will demonstrate a deeper understanding of the concept and show that you have considered alternative perspectives.

8. Summarize and conclude: In the conclusion, summarize the key points discussed in your essay and restate the importance of the concept. Offer your own insights or reflections on the concept, highlighting any implications or consequences it may have. Make sure to leave your readers with a clear understanding and a sense of the significance of the concept you have explained.

Remember, it is always a good idea to consult with your teacher if you have any questions or need further clarification on the assignment.