A digital Persistent IDentifer (PID, pl. PIDs) is a unique identifier that represents a something with a number or code. The number or code permanently and unambiguously identifies something in a way that is easily machine-readable. Machine-readable means that digital machines, computer systems, and the Internet find the codes easy to work with. The "something" being identified depends on what kind of PID we are talking about. Some PIDs represent digital objects such as articles, datasets, or grant documents. Other PIDs represent physical people or things, but in a way that is easy for digital systems to use.
Another name for these identifiers is "Digital Persistent Identifiers" (DPI or DPIs), because they are so essential to improving research systems in the digital environment.
Have you ever searched for one topic online, but the results were another topic that was just called the same thing? Maybe you clicked on an author's name, but for all of the authors with the same name. Or maybe you searched for an article listed on a reading list, and found several similar but no exact matches to the article title. Research, science, and scholarship are complicated; computers have trouble keeping track of all of these things. By using an appropriate PID wherever possible, you can make research communication systems better!