We are being asked to post “essential questions” in our classrooms this year. I’m not sure of what makes a question “essential” and how this would help students. Would I need a different question each day for my biology course?
—John, Boston, Massachusetts
My knowledge and experience with essential questions relates to the Understanding by Design framework from McTighe and Wiggins (see the end note). But there are other interpretations, so you should ask your principal what she has in mind. (Perhaps she could model this in a faculty meeting or professional development event?)
Basically, whether you use the term essential questions, big ideas, key understandings, or themes, the purpose is to focus student learning on important concepts that unite and underlie the lessons or chapters in a unit or course. They help students make interdisciplinary connections and see the bigger picture of science beyond the vocabulary and facts. Most models suggest using them at the unit level, rather than for every lesson.
Essential questions, big ideas, or themes provide a context for the topic and address “Why are we learning this?” During each lesson, students revisit the question, connecting new content or experiences with previous learning. For example, an earth science teacher I observed posed the question “How does the surface of the earth change over time?” As students investigated processes such as plate tectonics, erosion, deposition, or asteroid impact, she guided them to reflect on the question and record their connections in their notebooks.
I’ve seen teachers display the questions on the white board, on a bulletin board, in a PowerPoint, or on a flip chart. In some classes, students put them in their science notebooks.  The location should not be as important as how students use them.
Both the Next Generation Science Standards (NGSS) and A Framework for K–12 Science Education describe and focus on a limited number of core ideas and crosscutting concepts—the big ideas of science. As I learned more about the NGSS, I found examples of questions in the “Storyline” narratives on the website. These questions  could be adapted for your units.  For example, these are life science questions for secondary grades:
Performance Expectations by DCI [Disciplinary Core Idea]
From Molecules to Organisms: Structure and Processes

• How can one explain the ways cells contribute to the function of living organisms?
• How do organisms live and grow?

Ecosystems: Interactions, Energy, and Dynamics

• How does a system of living and non-living things operate to meet the needs of the organisms in an ecosystem?
• How and why do organisms interact with their environment, and what are the effects of these interactions?

Heredity: Inheritance and Variation of Traits

• How do living organisms pass traits from one generation to the next?”
• How are characteristics of one generation passed to the next?
• How can individuals of the same species and even siblings have different characteristics?

Biological Evolution: Unity and Diversity

• How do organisms change over time in response to changes in the environment?”
• What evidence shows that different species are related?

Performance Expectations by Topic

Structure, Function, and Information Processing

• How do the structures of organisms contribute to life’s functions?
• How do the structures of organisms enable life’s functions?”

Growth, Development, and Reproduction of Organisms

• How do organisms grow, develop, and reproduce?

Inheritance and Variation of Traits

• How are the characteristics from one generation related to the previous generation?

Matter and Energy in Organisms and Ecosystems

• How do organisms obtain and use matter and energy?
• How do matter and energy move through an ecosystem?

Interdependent Relationships in Ecosystems

• How do organisms interact with other organisms in the physical environment to obtain matter and energy?
• How do organisms interact with the living and non-living environment to obtain matter and energy?

Natural Selection and Adaptations

• How does genetic variation among organisms in a species affect survival and reproduction?
• How does the environment influence genetic traits in populations over multiple generations?

Natural Selection and Evolution

• How can there be so many similarities among organisms yet so many different plants, animals, and microorganisms?
• How does biodiversity affect humans?

Unfortunately, some principals may think of the questions as something to check off during a walkthrough. I had a principal who noted that while he was in my class, I did not address the essential question. I responded that the 10 minutes he was there did not include the beginning and end of the class, when we did indeed make the connections!
Additional readings:

• Essential Questions: Opening Doors to Student Understanding (including excerpts from the the new book by McTighe and Wiggins
• Understanding by Design
• What is an essential question?
• Essential questions
• Big Ideas in Earth Science 
• Why Do We Have to Learn This?
• Teaching the Big Ideas of Science