Question: I have just recently started using SQL Server Management Studio. After some hitches, I have managed to create a Database. Inside the DB I have also created some Tables and populate them. 

Now I am trying another software that needs to be connected to this DB. When running the software it asks me:

"Data Souce" and "Catalog"

Apparently the "Catalog" is the name of the DB I have created but I don't know what "Data Source" refers. 
It must be a rather begineer question but what does "Data Source" refers to?

Answer: 
It is free and a part of Visual Studio Community Edition.
The EULA of VS 2019 community edition has a specific section that allows enterprise users to use it for the SSDT workload. See <https://visualstudio.microsoft.com/license-terms/mlt031819/>.

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> Any number of your users may use the software only for Microsoft SQL Server development when using the SQL Server Data Tools or the extensions “Microsoft Analysis Services Projects”, “Microsoft Reporting Services Projects”, or “SQL Server Integration Services Projects” to develop Microsoft SQL Server database projects or Analysis Services, Reporting Services, Power BI Report Server, or Integration Services projects.
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<https://github.com/MicrosoftDocs/sql-docs/issues/1886>