Skimming is a purposeful preview. Look at the title, headings, introduction, conclusion and other signals of structure to work out what a text covers. You are trying to locate its main direction and the parts relevant to your question.
That first pass helps you decide where to slow down. It is not a substitute for reading the evidence or qualifications when accuracy matters. A useful skim ends with a better plan for reading, rather than a claim that you now know everything the text says.