Fair Use means you can use some copyrighted material, that is not public domain, legally, without paying fees for such use.
This is influenced by a number of factors and is rather complex!
From the US Copyright Office:
Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.
Transforming a copyrighted work into another work is typically ok;
Don't take any more than what you need;
Usually, using an item sparingly for educational purposes is ok;
Showing a copyrighted video to an in person class is ok.
If you're using an article or streaming video in courseware software (such as Canvas), use the database link to the material.
See also: Campus Guide to Copyright Compliance.