range of coverage

Hi PP gurus

can you explain the usage of range of coverage profile in MRP run as well as how the requirements will cover for the future

explain the settings to be required in master data as well as in configuration needed

In my scenario the stock is 0 qty but it is showing as 21.0 days as coverage profile . i have requirements as well as receipts are availabe for the same material can any one explain how it is getting calculated as range of coverage as 21 days

thanks

Shaik