Public domain is the term used to describe works that are no longer under copyright protection. Copyright is granted for a limited time and that time period has been extended several times since the 1970's. Currently, most copyrights last for the life of the author plus 70 years. At the end of the copyright period, the work becomes available for general use.
Works published before January 1, 1923 are in the public domain and not governed by copyright restrictions. If the work that you wish to use was published before 1923, it is in the public domain and you are free to use it. However, if the publication date is after 1923, there are several things to consider. What is the date of publication? Was the copyright renewed? Do you know who owns the copyright? Does your use fall under the Fair Use provision?