Plugins, also called add-on or extension, is computer software that adds new functions to the main program without altering the main program.
Plugins enable programmers to update a host program while keeping the user within the program’s environment.
Plugins are addendum functionalities that provide more features or functionality to the host program without altering the software architecture or the core functionality of the host program.
WordPress Plugins are microprograms that perform a definitive task based on the requirement and necessities of the website owners.
Analogy: Factory
A factory uses many assembly lines, which are manufacturing systems in which production, which is a work-in-progress, moves from station to station in a sequential fashion. These stations are assembly lines. An assembly line is a production process that breaks the manufacturing of a good into steps that are completed in a pre-defined sequence.
Similarly, WordPress is a factory that produces webpages, where its database is a raw material. WordPress assembly lines are the PHP files and PHP template files of its core and WordPress Themes.
But all factories outsource jobs and bring in contractors to add value and perform additional task-specific operations, which are addenda to a pre-defined sequence of different assembly lines.
Similarly, WordPress plugins are those outside contractors, which on top of WordPress core features performs certain additional functional tasks to enhance the production of WordPress pages/posts by increasing their performance, feature, and functions.