The Technology Education and Copyright Harmonization (TEACH) Act refers to changes made to Section 110 of the Copyright Act that describe the conditions under which instructors and students may perform and display copyrighted materials in a virtual or digital classroom without obtaining permission from the copyright owner. These are bright-line rules; if your proposed use meets all the TEACH Act requirements, permission is not required to use the work. If your proposed use does not meet all the requirements, the use may still be permitted without permission of the copyright holder under fair use.
A key point to remember is that the TEACH Act is intended to cover classroom-type instruction delivered digitally, including over the internet. It does not cover materials an instructor may want students to study, read, listen to or watch on their own time outside of class, including electronic reserves. For these uses, the instructor must continue to rely on the principals of fair use.