Creative Commons Licenses offer an alternative to the "All Rights Reserved" model of intellectual property that is created by default when a work is expressed (and thus falls under copyright law). The copyright holder can choose to publish their materials under Creative Commons licensing to increase its usage (and some would argue that this increases its value), to allow for collaboration with other content creators, and/or to meet aspirational goals of contributing to the "greater good" of society. It provides the licensing basis for "open educational resources", including "open access journals".
For educators who wish to use photos, music or scholarly literature, it is advantageous to opt for content that has a Creative Commons license. That generally means the content is not subject to traditional copyright restrictions. Before using content with a CC license it is important to take note of the type of license and adhere to the wishes to the license holder.
An alternative to copyrighted content is open education resources (OER). OER often has a Creative Commons license. The availability of OER textbooks will vary between disciplines but there are many other types of OER learning content. A good starting point for learning more about OER is the Library's research guide on OER.