A Plugin is a set of files for specific functionality. People create WordPress plugins to perform a function on their own websites or to upload to WordPress directory for public. Suppose Yoast plugin is a set of files to create a function of adding meta tags and other SEO tags to google. It is programmed on Yoast's set of programming files to add meta tags to each post which you can't add by yourself. For learning purpose you can download any plugin from WordPress.org and unzip it and read their coding files, you should have PHP skills to read. Plugin simply add a feature to your website. Suppose you visited a website, and that website have a print option, People can print articles using that feature. Than you can add that print option to your website and you don't need to write whole for it. Just go to plugins in your WordPress admin area, and search print. There you will see thousands of plugins, may be some are with same name. Find plugin which is suitable for you and just install and activate.