Composer is a tool for dependency management in PHP. It allows you to declare the libraries your project depends on and it will manage (install/update) them for you(refer link.
Composer is the dependency manager for PHP that allows you to add and manage script libraries and packages as dependencies in your Magento website, in order to reduce development time and have the ability to take advantage of third-party code.
It helps us installing/updating various requirements/components for Magento core and extensions updates.
