A variable is a symbol that represents something else. You might use the variable product_name so that if you're product name switches, you don't have to update all instances of the product name in your documentation. You can just update what the variable product_name points to.
DITA doesn't have an actual variables element, but you can use keyref to accomplish the same thing.
In your map file, add the following:
In your topic file, reference the variable with the following:
You can event put attributes on the keyword so that different product names appear based on different conditional processing.