All document entry, whether it is Sales Invoice or AR Payment, they consist of at least 2 DataTable, master table and detail table.
Some may have more than 2 tables, such as AR Payment.
Usually the master table is where it stores the document information, such as document number, document date, company name and etc...
Whereas, detail table stores multiple records that are related to this document. Such as Items and account double entry transactions.
MasterColumn refers to the value of a field in master table.
Likewise, DetailColumn is referring to the value of a field in detail table.
Invoice Entry Form This screen shows there are two highlighted areas in Green and Red, which are master and detail area respectively.