You are able to add merge fields in your campaign, as well as add any personalized fields you might have set up for a subscriber list. When adding a custom field, you can specify a personalization tag for that field. Whenever you add these tags to a campaign and send it to that list, those fields will be populated with the subscriber's information.
Here is how it works:
Say that you want to add a fallback field (that is, a field that will be populated if the field the data comes from is left empty for any subscribers:
[field name,fallback]
Therefore:
[firstname,there] - would pick the field [firstname] and populate it with the information available on your list, and if empty, would use there instead.
So Hi [firstname,there], could read: 'Hi Alex,' or 'Hi there,'