Skimming is reading a text quickly to get a general idea of meaning. It can be contrasted with scanning, which is reading in order to find specific information, e.g. figures or names.

Example
A learner taking a reading exam decides to approach text by looking at the title, introductions, and any diagrams and sub-headings, then skim reading to get a clear general idea of what the text is about.

In the classroom
Skimming is a specific reading skill which is common in reading newspapers, messages and e-mails. It is important that learners understand that there is no need to read every word when skimming, so often teachers set this as a timed task to encourage speed.

See also:
https://www.teachingenglish.org.uk/article/scanning

Further links:
https://www.teachingenglish.org.uk/article/teens-reading-skills
https://www.teachingenglish.org.uk/article/theories-reading-2
https://www.teachingenglish.org.uk/article/unit-4-reading-skills
https://www.teachingenglish.org.uk/article/making-reading-communicative
https://www.teachingenglish.org.uk/article/activities-using-magazines-classroom