What is Personalization variables (merge fields)?
Placeholders like {{first_name}} and {{company}} replaced per recipient from your lead data, with fallbacks for missing values.
Variables pull a value from the lead's record into the message. The detail that separates working sequences from embarrassing ones is the fallback: {{first_name|there}} sends "Hi there" when the first name is blank instead of "Hi ,". Any column in your CSV can be a variable, which is how specific, researched lines get mail-merged without looking mail-merged.