You must have observed this type of feature on many websites. By default, WordPress allows you to select taxonomy terms one by one, even if you select any parent taxonomy, you have to select other child taxonomy terms one by one. This is good for most WordPress users but sometimes you need more options.
This tutorial is on new WordPress plugin Select All Terms,the plugin is very easy and simple to use. There are no options to setup. Simply install and activate the plugin. Upon activation the plugin automatically inserts two buttons at the hierarchical taxonomy metaboxes (like categories) to ‘select all’ or ‘deselect all’ terms.