Before we can get into Creative Commons licenses, we need to be sure have a basic understanding of copyright.
According to the United States Copyright Office, "copyright is a form of protection provided by the laws of the United States to the authors of 'original works of authorship' that are fixed in a tangible form of expression." An original work is something you yourself create that requires some level of creativity (original thought). It could be written word, music, a play, art, a movie, etc. You do not need to register your work to hold copyright for it; rather, you automatically have copyright the moment you create the work in a tangible form. So what's a tangible form? As with most elements of copyright law, there are some gray areas, but certainly anytime you post a work online for others to view (as you likely will in this class) it becomes tangible. (More on that, if you are interested.)
Copyright gives exclusive rights to the copyright holder for a set period of time: The right "to make and sell copies of their works, to create derivative works, and to perform or display their works publicly" (Wikipedia). (The term "all rights reserved" refers to these rights.) How long is that period of time? It depends, but under current copyright law it is most often 70 years after the death of the author.
According to the United State Constitution (Article 1, Section 8, Clause 8), the original purpose of copyright law was "To promote the Progress of Science and useful Arts..." Put in different words, it was intended to "[encourage] systematic knowledge and learning of enduring value" (L.B. Solum, p. 54) or encourage the sharing of ideas and knowledge. When ideas and knowledge are shared, others are able to not only utilize them but also to build and improve upon them to society's benefit. It was hoped that by reserving certain rights for copyright holders, it would encourage them to share ideas and knowledge.
Over time, copyright laws have gotten more and more strict and convoluted (especially in terms of how long a work is covered by copyright; thanks in large part to Disney), to the point where they are much more effective at preserving copyright holders' rights and profits (even though the vast, vast majority of works -- like the papers you write for class, most of the journal articles you find through the library, and certainly your social media posts -- generate no profit for the author) than they are at stimulating the sharing of ideas for the greater good. Applying a Creative Commons license to your work allows you to stimulate the creativity of others and foster the sharing ideas.
Copyright law allows for some use of works without requesting permission from the copyright holder. This is called "fair use." If you're interested, you can learn more about fair use and determining whether an intended use may be considered fair under the law in this Columbia University guide. You may also be interested in this short video.
In addition to being quite limited, a big problem with fair use is that it isn't black and white. Ultimately, whether or not something is truly a fair use in a specific case must be settled in a court of law. Applying a Creative Commons license to your work not only expands what people can do with your work without getting permission but also clarifies what they can and cannot do.