Offset is a parameter that you might find associated with your camera settings alongside gain. It’s not something that’s used outside of astrophotography so you might not be familiar with it. To explain what it does, you should first look at a histogram curve for one of your exposures. You’ll see that the data is crammed to the left hand side of the graph. The offset settings allows you to artificially move the data to the right in the histogram so that the bottom part of the curve does not clip. It ensures that you keep all the data without any being dropped. The actual offset value to use depends on your equipment but “30” is an often quoted value to be used as a starting point. Dale Ghent’s article explains in detail how to optimize your offset.