WordPress Multisite is a feature of the WordPress platform that allows users to create a network of multiple sites, all running on a single WordPress installation. It allows users to manage multiple sites from a single dashboard, and to easily share data, users, and content across sites.
Databases, user accounts and server resources are all shared in-between all websites you have tied to the WordPress Multisite feature. It will make your life easier to single-handedly install plugins and updates to all sites. However, there is also a chance that if there is an error or a crash, all sites can go down together.