Merge tags fill in each lead's details when the email is sent.
Merge tags and fallbacks
Write {{firstName}} to use a lead's first name. Add a fallback after a bar for leads where the field is empty: {{firstName | there}} becomes "there" when there's no first name. Any custom field from your CSV works the same way.
Variety
Write {{RANDOM | Hi | Hello}} and each email picks one option, so emails read a little differently.
Check before you launch
Preview the email for any lead in the editor to see exactly what they'll receive, and send yourself a test.