When we describe, we note physical and sometimes chronological details. Descriptions generally rely on sensory perceptions (compared to "analysis" that typically gets at mental abstractions). Because vision is usually our dominant sense, most of our descriptions rely heavily on visual details. For many essay questions, being asked to "describe" means writing about what you've seen.
Writing tip: Although our field of vision takes in lots of details, we organize those to help remember them. As writers, we need to make our organizational pattern obvious to readers. That's why most descriptions follow a top-to-bottom, right-to-left, etc., consistent pattern of moving over a visual scene. Sometimes, the pattern is most-to-least important, and this pattern works especially well if your description is building to a particular point.
Depending on the situation in which you are asked to "describe," you may want to organize the details of your writing according to a chronological pattern. Particularly when you are recording observations that take place over a long time, you may want to capture the sense of passing time by using time markers (e.g., first, later, finally) to organize the details in your writing.
Specific advice for OT students: You are working with models of assessment that ask you to note certain kinds of physical movement or reactions in a certain order. When the model of assessment has a built-in order, you can use that to organize the details of your description.
Substitute key words: observe or notice