It is important to note that any changes that you make directly into your plugin’s core files will be overridden when you update the plugin. In theory, you should never be using the plugin editor to edit any files. Often developers use the built-in editor to view the code to make appropriate changes by utilizing a hook or filter within the plugin. If for some reason you find an absolute need to edit the plugin’s core file, then we recommend that you talk with the plugin author first. Perhaps there is a better way of accomplishing what you are trying to do rather than modifying the core plugin file. The only reason why we say this is because you don’t want to spend the time to make the modifications every time you update the plugin. At the same time, you don’t want to stop doing plugin updates because that could put you at a serious security risk should there be a vulnerability in the plugin.

Unlike the the Theme Editor, if you make a syntax error in your plugin editor, then it won’t lock you out of your WordPress admin. It will simply deactivate the plugin and explain the error to you.

Lastly, you should only use this section if you know what you are doing.

Source: WPbeginner Glossary