Stanford's resources, written by a lawyer, offer basic definitions as well as historical context. They also keep a running list of examples of precedent-setting fair use cases and how they were decided.
https://fairuse.stanford.edu/
https://fairuse.stanford.edu/overview/
For those looking for more in-depth resources, especially those designed for librarians and information professionals, ACRL's Scholarly Communication Toolkit has a helpful list of publications and courses.
https://acrl.libguides.com/scholcomm/toolkit/copyright
Copyright.gov is an authoritative and regularly-updated resource. It includes some information on the impact of COVID-19 on Fair Use. The actual laws are posted on the https://www.copyright.gov/title17/ page, broken down into sections.
https://www.copyright.gov/