This setting adjusts the flow rate for the support.
Support interface refers to a flat roof (and/or floor) on support structures that better conforms to your model. It tends to improve surface finish and break away easier, assuming you don't accidentally fuse it to the print.
The "interface" means the interface between the model and the support. It has to touch the model, not the build plate.
Support roof refers to the skin at the top of the support on which the model is printed.
Support floor refers to the skin at the bottom of the support, where it rests on the model.
Only when the support skin rests on the model (right side in the figure below), it can be called support floor. If the support is comes directly from the build plate (right side in the figure below), then it is not a support floor.
The flow rate for the support can be adjusted separately from the flow rate of the rest of the print.
Adjusting the flow rate during the support is a stop gap method to fix problems with extrusion rate or the adhesion between the support and the print. The same effect can be achieved by adjusting the line width or line spacing of the support, but adjusting the flow may be more intuitive.
If there is a problem with extrusion rate or strength during the support, it is better to look at the printing speeds and temperature. Perhaps the flow difference between the support and the other structures is too great to properly extrude. Another common issue with the flow rate of support is that there are too many crossings in the support pattern. It could help to choose a pattern that doesn't cross itself, such as Gyroid or Zigzag.