'Wel' doesn't really have an easy literal counterpart in English. It is used to stress the contradiction with 'niet':
- 'Het is waar': it is true
'Het is wel waar': it is true (something you would say when someone is implying it is not true)
'Heb je wel of niet de post gevonden': did you or did you not find the mail?
- 'Ik heb de post wel gevonden': I did find the mail.