Links field are the ones linked to another document type. For example, customer field is a link field in Sales Order. This field is linked to the Customer master.
You can insert Custom Link Field by following steps below.
In Customize Form, select Document Type (Quotation, Sales Order, Purchase Invoice Item etc.). Once fields are updated in the accompanying table below, open a field above the one you wish to insert your Custom Field. Then click on "Insert Above" to insert the new Custom Field.