When a shortcode is inserted in a WordPress post or page, it is replaced with some other content. In other words, we instruct WordPress to find the macro that is in square brackets ([]) and replace it with the appropriate dynamic content, which is produced by a PHP function. Shortcodes mean shortcut.

WordPress shortcodes were introduced in version 2.5 and since then have proved to be one of the most useful features.

Sources of More Information

Smashing Magazine has a detailed guide on how to write a shortcode yourself.
https://www.smashingmagazine.com/2012/05/wordpress-shortcodes-complete-guide/

You can install a WordPress plugin called Shortcodes Ultimate. It enables you to easily create buttons, tabs, boxes, sliders, tooltips and many more elements. It is one of the best shortcode plugins out there.

The J Shortcodes plugin is similar to Shortcodes Ultimate, offering a collection of useful elements to style a website, including buttons, boxes, tabs and accordions. J Shortcodes lets you set custom attributes on elements, such as color, size and shape, and define custom column layouts on any page or post.