A very common customization is adding of custom fields. You can add Custom Fields in any Master or Transaction in ERPNext. To add a Custom Field, go to:
Setup > Custom Field > New Custom Field
In the form:
and save the Custom Field. When you open a new / existing form of the type you selected in step 1, you will see it with the Custom Fields.
Many times you want your fields to be carried over from one form to another. For example, you may have added a Custom Field in Quotation that you want to include in Sales Order when a Sales Order is created from the Quotation. This is simple in ERPNext, just make sure the fields have the same “fieldname”