Comma Separated Files are the normal way of getting data into PBi
Depending upon who you are and what sort of system you have, you either create one yourself from your admin system or you get your "IT bod" to do it for you from your database.
As a generalisation the more modern the computer system that holds your data is, the easier it is to get the data out in a format suitable for PerBI or any other product.
In the past we have done things like a "mail shot" to get the necessary data, as well as more modern things like SQL queries.
It is possible that the only way to get data out of your system is to go back to your supplier and ask for a program to do it, this is more likely to be the case if you have had a bespoke application created.
What Is A DIS File?
When you use PerBI, the first thing that it asks for is a DIS file. A DIS file is simply a
Data
Import
Specification and it describes what's in the data file.
Making a DIS file can be as easy as Selecting the menu options
DIS->Autobuild.
A DIS file is not a "metacube" or anything like it, it's much more like a permanent record of what Excel asks you when you open a text file rather than a spreadsheet file.
You have to specify a DIS file as the application can make best guesses, hence the autbuild, but this is not always how you want to interpret the data.