I ll try make it as simple as possible for you in here ..
When you skim you search for what you want or need out of the text .. Like you read any questions that you need to answer before you go reading the text then you ry to look at the text fast searching with your eyes for the keywords that are related to your question .. And hence you can get to your answer fast ..
As for the scanning thing it is more like super fast reading that you do by simple trying to go through the lines all of them as glimpse reading to figure out the main idea or topic of a paragraph or a chapter ..
Best Luck!
Wise
