Master detail is a way to filter grid with spesific ID from master grid . Its work like shortcut button.
Ilustration you have 2 grid table , and the 2st table contain relation key from #1 table. so you want to add additional button from #1 table to manage grid with selected ID from #1 table.
Edit your module and point to Master Detail Tab :
As you can see from image above , this setting is from your 1# module/table . Once u done saving master detail setting dont forget to rebuild module ( all files )