You can personalise your messages according to any information imported into the system. A common personalisation is by contact name, like “Dear John”.
For personalisation to work, the contact needs to have been imported with that field. If the personalisation tag is inserted correctly into your email, but the contact field you’re using to personalise it is not populated, it won’t pull through successfully. E.g.: It isn’t possible to send an email to your contacts using first name personalisation if your contacts were imported without the first name as a field.