It is in fact a Relational Database Management System (RDBMS), which is basically a fast and high-performing database which allows users to create websites based on highly professional database.
SQL Server 2017 offers a reliable, productive and efficient data platform to run the most demanding mission-critical applications, cut down on time and development and management costs of the applications and provide information translatable into action on all levels of the organization. The interaction system is related to Microsoft platforms (on Windows system with ASP or ASP.NET technology), Linux and Docker.
The MSSQL Database is necessary to be able to use the main CMS that rely on this type of database.
Starting from 10€ + VAT/year you get a database and disk space from 1GB. You can also choose to purchase a plan with more disk space (up to 10GB), at 10€ + VAT for each 1GB.