The last Challenge was to get a photo of your subject at the longest focal length you can, at the widest Aperture you can, and at the closest focus distance your lens will allow. What did you learn from that experience?
You certainly learnt about minimum focus distance. Did you try it on different lenses? At different focal lengths? It will be different.
We can think of Depth of Field as the area of the photo that is in focus. You have already noticed that almost all photos are out of focus in the foreground, and out of focus in the background, even if the background it far away. It's actually quite hard to get everything front-to-back in focus.
The things that determine how much is in focus - or the Depth of Field - are exactly those you practised with in your Challenge: Focal length, Aperture and focus distance.
General rule: The longer the focal length, the wider the Aperture and the closer the subject, the narrower the DoF is going to be.
There is maths you can do with pen and paper, and apps your can install to calculate what your DoF is going to be in any given situation. And there's a website for that: http://www.dofmaster.com/dofjs.html
So for example, if you have a 55mm lens, at f5.6, with the subject at 1m away from you, only 7cm is going to be in focus. Everything else is going to be blurry.
I made the amateur mistake a few weeks ago while taking a portrait of my sister and brother-in-law. I was shooting at f4 because I wanted to blur out as much of the messy background as possible. I was using my 24-105mm lens, at the long end, and they were standing about 3m away. My error was that I set focus on my sister, but didn't realise that brother-in-law was standing just fractionally forward. So sister was perfectly in focus. Brother-in-law's hair and collar is is focus, but his face isn't. The 12cm of focus I had to work with covered sister's nose to ears, but nothing in front of that. A reshoot is necessary - what an embarrassing mistake! What should I do next time? (Hint: find a clear background with no distractions.)
Can you imagine trying to get a photo of something with an 85mm lens, at f1.4, that is just 1m away. With a bit of luck and steadiness, your subject's eyeball could be in focus, but their eyebrow not.
At the other end of the spectrum, what about a photo at 24mm and f8, where the subject is 10m in front of you?
What about 50mm at f2.8, with the subject 20m away?
When talking about Depth of Field, you may hear the word "bokeh". Bokeh is the quality of out-of-focus areas, as opposed to the degree of defocus. Bokeh is pronounced bo-keh - sort of like "bouquet", but shorter and "eh" at the end.
You'll need three subjects, or a subject and two other similarly sized objects for this one. Position them one behind each other, about a metre (or less) apart. Not necessarily in a line, but so you can see them all in the frame. Using a similar technique to last time, make sure only the one in the middle distance is in focus (Shallow Depth of Field).