When we talk about scanning a text we make reference to a quick reading just to find specific information. This technique is regularly used with students need to read a text and article, etc and try to answer certain specific questions or maybe do a true or false activity.
You might scan a dictionary to find a word, a catalog for a determined product, etc.
Dictionary definition: "Scanning a text is a reading technique where the reader looks for specific information rather than trying to absorb all the information"