Hi trungtran,
A "Linked Field" is something you would add under Customize > Tables / Fields. You would click on Edit Fields... for the Opportunity table and add a linked field using the wizard, linking it through the Contacts dropdown.
Once the information you want is in the Opportunity table, you can add it to the New Opportunity and/or Edit Opportunity screen in the "Contact Information" section.
Hope this helps.
- Justin