What you're looking at here in the picture above are
concrete footings that will be supporting a stairway. My personal
opinion is that these footings are a little excessive and over
for the weight that this set of stairs is going to support.
However, since I'm not a structural engineer, what would I know?
By now you should have a general understanding of how the stairway can or should be assembled. If you're building a set of stairs on the exterior of your home, for a deck or porch, then it would be a good idea, to make sure that the stairway and supporting structural walls have a nice thick concrete footing, to support them.
Now, this is where most novice construction do-it-yourselfers run into problems. Figuring out exactly where to locate the bottom stringer connection footing and supporting walls footings can be tricky.
The best advice I can give you, would be to figure out your total rise and run and then cut one stair stringer. And then you can place the stair stringer into position, to see exactly how wide the footing needs to be and where it would need to be located.
Of course this method only works if you don't have an existing stair stringer bottom connecting footing. The positions of these footings can also be figured out, by using a little common sense and a measuring tape.