Depth of field is the area you see in the frame that will be IN FOCUS. The smaller the aperture, the larger the DOF, most everything in the frame will be in focus. As you open up the aperture, less and less of the frame is in focus, leaving the rest blurred.
More interestingly; the instructor pointed out that when you are taking a GOOD picture, you need to determine "what's the focus"? if you are trying to get the audience to look at a certain point, you would want to open your aperture, blur the rest and the focus becomes what you we're seeing through the lens. fascinating. In my layman thinking, I would have thought you would want everything to be clear, but I am now "educated", which was the point.
My homework assignment was to take two pictures, one with small DOF and the other with large, or wide.
I have spent a WEEK trying to "find" the right opportunity or "moment" for this; it finally happened this morning when Ethan was checking to see if his watermelon plant was sprouting yet.... if I do say so myself; COOL!