As a general rule, copyright is automatic. In the U.S. a work is protected by copyright once it's fixed in a tangible medium, when its embodiment in a copy or recording, "by or under the authority of the author, is sufficiently permanent or stable to permit it to be perceived, reproduced, or otherwise communicated for a period of more than transitory duration." (17 U.S. Code § 101 - Definitions, from the Cornell Law School Legal Information Institute)
Copyright grants exclusive rights for copying, distributing, publicly performing, adapting, or otherwise using a work to the copyright holder, often the creator of the work. Others wishing to copy, adapt, distribute, perform, or otherwise use the work must obtain permission from the copyright holder.
Copyright grants rights to work that is original, a creation not copied from another work. Some examples:
Copyright does not protect facts or ideas, only the expression of those facts or ideas.
“Copyright Basics” (https://certificates.creativecommons.org/cccertedu/chapter/2-1-copyright-basics/) by Creative Commons. CC BY 4.0.