Skim reading means reading quickly through a piece of reading text so that you can get the overall gist of it without reading every word. You might read the title, headings, subheadings and the first sentence of every paragraph. Any pictures or photos that appear with the text can also help you to get the gist of what the text is about.
Skimming is useful for helping you to identify the main ideas of a text and to find out whether the content is of interest to you.