Need an explanation of fill an offset fill

Fill - uses parallel linear scan lines to fill the area of a shape. This is the general purpose fill method and is perfectly appropriate for pretty much all jobs and generally results in the best quality.
Offset fill - fills the area of shape using lines that follow the outline of the shape. While not generally applicable to all jobs, this approach can save time, especially on slower jobs where movement in unfilled whitespace can take a large proportion of job time.