I think this essay is all about the science: from the focus of your lab as a whole, to the scope and aim of your particular project(s), to your particular experimental design and execution on said projects, to results and analysis of your data. You could probably get as technical as discussions during lab meetings or presentations at conferences, but I wouldn't go out of my way to dumb it down. Also discuss any presentations, pubs, or grants you received or contributed to. I would aim for clarity and assume the lowest education of someone in the audience is that of yourself. I'm pretty sure most, if not all, people that read it will either be MD, PhD in some kinda science, or both.