The first Mishna in the perek (see 2a) taught that a sukka whose walls are more than 20 amot high is not a valid sukka. The Gemara on our daf teaches that if the walls are too high, it can be rectified by building a platform, extending from one wall to the next, that is, by itself, large enough to be a valid sukka. By doing this, we effectively lower the walls of the sukka to less than 20 amot.
The Gemara follows this ruling with three other examples of cases where a sukka whose walls are not the right height can be fixed by manipulating the height of the floor:
When a platform is built on one side of a sukka that has three walls taller than 20 amot. In this case, the platform must reach to within four amot of the opposite wall so that that wall will be considered part of the sukka.
When a platform is built in the middle of such a sukka. In this case there must be less than four amot between the platform and the walls on both sides so that those walls will be considered part of the sukka.
If the sukka was less than ten handbreadths high and he dug out an area inside the sukka in order to complete the requisite height of the sukka to ten handbreadths, if from the edge of the dug-out area to the wall there is a distance of three handbreadths, it is unfit, as in that case the edge of the dug-out area is not joined to the wall of the sukka. Therefore, even though the interior space is ten handbreadths high, its walls are not the requisite height to be considered a fit sukka. If the distance from the edge of the dug-out area to the wall was less than three handbreadths then it is fit.
In any case, dofen akumah is one of many legal fictions that are permitted by the Sages in creating valid walls for a sukka.