What you're really asking is: "how does a compressor work?" -- there are lots of great YouTube videos that explain the basic concepts, including all the controls in Fruity Compressor, so I would start there.
If you like learning by experimentation, though, it might be more instructive to use Fruity Limiter instead (using the compressor controls by clicking the "COMP" button at the bottom of the window) because it allows you to see visually over time how it reacts to an input signal.
In general, for compressor plugins that don't have "multiband" in their name, or do have "wideband" in their name, that means they work such that the compressor's gain reduction applies to the entire frequency spectrum, just like a regular volume/gain control does. A multiband compressor (in FL, Fruity Multiband Compressor and Maximus) applies gain reduction to certain ranges of frequencies independently from each other, which is more like applying an EQ to the signal.