Personalization means that each recipient gets a separate email with slightly different content, the typical example being:
To use this feature, you first have to make it available in the Settings form, tab Features :
Below the Text editing area, a combo box appears containing fields in the mailing list, with an [Insert Field] button
With the Built in mailing lists, personalization is limited to the [fullname] field. If you want more fields, you will use either Linked table mailing lists or Imported table mailing lists and make sure the additional fields required are available in them.