First let's work out the problem, which gives you the following information:
=> you have a board that is 12 feet long to use to make the steps for the set of stairs
=> each step is 3.5 (or, 3 1/2) feet long
To see how this problem plays out, imagine cutting the first 3.5 foot long step. Now you are left with a board that is 8.5 feet long (12 ft - 3.5 ft = 8.5 ft). Then you cut the second 3.5 foot long step, and are thus left with 5 feet (8.5 ft - 3.5 ft = 5 ft). Cutting the next 3.5 foot long step leaves you with 1.5 feet (5 ft - 3.5 ft = 1.5 ft). Since you are now left with a board that is 1.5 feet long, you cannot make another "complete" step that is 3.5 feet long like the others. Thus, from a 12 foot long board you can only make 3 complete steps which are 3.5 feet in length and you are left with a remainder of a 1.5 foot long board. That is what the word "complete" is referring to in the context of this question.
An easier way of doing the math behind this is by simply dividing the 12 foot long board by the desired length of each step, which is 3.5 feet long....
... i.e., 12 ft / 3.5 ft = 3.42857 ≈ 3.4
If you separate this yield of 3.4 into 3 and 0.4 ( 3 + 0.4 = 3.4 ), you find that you can make 3 whole or complete steps and one incomplete or a fraction of a step.