When evaluating web sources, "lateral reading" ("lateral" meaning "side-to-side") refers to the practice of seeing what other sources have to say about the source in question, rather than seeing what it says about itself, or "vertical reading" (e.g. by reading the "About" page of a website). Often a simple Google search is enough to establish whether a site or organization has a reputation for sharing unreliable or heavily biased information.
Read more about lateral reading here: https://webliteracy.pressbooks.com/chapter/what-reading-laterally-means
Note: lateral reading can be applied to scholarly sources as well! Remember that "peer-reviewed" does not always mean "reliable information." It usually does, but sometimes peer-reviewed articles are later retracted for inaccuracies.
Lateral reading activity: https://webliteracy.pressbooks.com/chapter/activity-evaluate-a-site/
Stop
Investigate the source
Find better (or additional) coverage
Trace claims, quotes, and media to their original context
Adapted from "Check, Please!" (Caulfield) at http://lessons.checkplease.cc (CC-BY)