It looks like nobody answered the original question.
The step over percentage represents the percentage of the bit diameter that will cut on each pass. Lower percentage means less material will be removed on each pass, hence longer machining times.
There is a balance to achieve, too high a step over will increase jitter, and overheat the bit. Too low of a step over will make the cutting edge burnish the material, also increasing the heat and reducing bit life.
Look at Inventables youtube videos on bits 101 and 201, they do a pretty good job on basic understanding of the various settings.
Hope this helps!