Let is say you have value you want to display in Ax report or Form, and this value not available in your table for example
We have salesTable in AX have salesStatus field and you want to display sales order status in your report or form, to achieve that you want to create display method in SalesLine Table to display sales order status in required form and report.
Display salesStatus getSalesStatus()
select salesStatus from _salesTable where _salesTable.salesId == this.salesId;
return _salesTable.salesStatus ;