To answer that, we have to talk about Copyright for a few minutes. (Bear with me.)

So, 'copyright', also know as 'author's rights' is actually a bundle of several rights, to reward creators by allowing them an exclusive (if temporary) right to profit from their intellectual property:

Economic - to make $$$, of course, by way of:

• The Right of Reproduction:
• The Right of Publication or Communication to Public
• The Right of Performance
• The Right of Audio-Visual Expression
• The Right of Translation and Adaptation

&

Moral:

• The right to known as the author of a work, and to 'integrity', to keep others from damaging or misusing their work in such a way that the author's reputation suffers.

So an author has the right to manage their copyrights as they see fit. They can keep all these copyrights to themselves ('all rights reserved'), or can let other people use their rights - all of them, or just one, or any combo.of the individual rights.

Now, the way you legally give someone permission to use some of your copyrights is to issue a license to them (legal permission to do something - think of drivers licenses granted by the State). Normally, authors charge money for these license agreements, such as making a deal with a publisher who will print and distribute their book, or with a movie producer to transform it into a movie.

But what happens if an author doesn't really care all that much about making money from their IP?

Then, in latter few decades of the 20th century...this amazing new digital publishing platform came along. It was free to use (or close enough as no matter to many), and people immediately began to share information by uploading it to internet web sites. Proponents of the 'Open' philosophy (which among other things, loves the idea of free, transparent access to information) liked this trend very much.

A non-profit NGO called Creative Commons formed in the early 21st century and asked the question: The legal way to share your copyrights with others is a license.- what if we made it easy and painless (no communications, negotiations or lawyers needed!) for authors to let the public know that they were voluntarily sharing some of their copyrights (but not all!) with the public for free. And so...Open Licenses were created.  They are:

• Genuine Legal license documents stored on the Creative Commons website
• A plain-language translation for laypeople
• Machine code to add to the html files so that internet search engines would know this uploaded item was openly-licensed
• Free to use
• A group of six open licenses. See them on the Creative Commons site.

Creative Commons also created visual, logo-style elements to be added to online materials so that there would be an especially quick way to visually identify the item as openly-licensed, including which of the 6 licenses.

So, to sum up so far:

• Authors own a bundle of several copyrights
• They can do with them what they wish
• Some people want to make their work available for free, on the internet
• The legal way you do this is to license it
• Since the early aughts, 'open licenses' that are used to label internet-published works exist
• Open licenses let the public know, up front and without having to ask, what the terms of use of the work is
• Nonprofit Creative Commons created 6 open licenses for pretty much everything except software (open software community already had their own)
• There are 6 licenses because authors deserve choices as to how they manage their IP, and not everyone wants to give up the same right or rights
• The one right they NEVER give up with these licenses is the right to be known as the author of the work (aka: Attribution)
• These licenses are used for writing, music, art, drama, films, etc..  And, of course....textbooks and other teaching materials.

Lots, actually...Lots.

Four out of the six open licenses created include terms that let the public change the work. Literally take it and...change it. Edit it any way that you want.

Remove some stuff, add other stuff.  Take another openly-licensed work and remix it into the first openly-licensed work. Add a random chapter from yet a third openly-licensed or public domain work. Outdated information? Fix it. Nothing but pictures of white people? Swap some out for images way more representative of today's diverse students. Ignoring the increasingly-recognized past (and present!) scholarly, inventive, and scientific contributions of BIPOC, women, and LGBTIA+ folks? Update that, too. Write and add in some of your own material? Absolutely. Make the material as accessible and UDL as you currently can? Excellent idea!  If and when you have created a new work (you used and will attribute others' works, but the end result is transformative and original enough for YOU to be able to openly-license it now), you can pick an appropriate open license for your new creation. Put it forth into the world...and someday others may use, modify, and update your work!

THE SKY'S THE LIMIT.

Oh, and you can do this thing called 'Open Pedagogy', too.