Joomla gives you the ability to make this changes in a easy way, instead of changing the core files you can move the view / layout files and make the required changes.
What is a Layout Override and why do you need?
Very shortly an override is a file that is loaded instead of the normal core file of an extension.
When you are creating your website with Joomla, you may want to change the output of the core Joomla Extensions and also the third party extensions that you want to use for your project. For the 3rd Party Joomla Extensions, you must be aware that the extension must comply with the MVC design pattern.
Overrides provide extra power and flexibility to web designers and site builders, who may only be concerned with creating new designs rather than manipulating the underlying code.
Which type of extension outputs can be overridden?
You can override Component, Module and Plugins which is coded and comply the MVC Design Pattern. You may also override the language file.
Override files are always placed inside your template's files.
In the next tutorials you may read Component Layout Override, Module Layout Override, Plugin Layout Override and Language File Override.