A merge tag is a placeholder that gets replaced with each reader's own data when the email sends. "Hi there" becomes "Hi Sarah" for Sarah and "Hi Marcus" for Marcus — from one email.
Insert a tag
While editing a text block in the email builder, insert a merge tag for the field you want — first name is the most common, but you can use any field on the reader's record, including the custom fields you imported or collected through a form.
Always set a fallback
Test with real readers
Send yourself a test and preview against a reader who doesn't have the field set, not just one who does. The empty case is the one that embarrasses you in front of your list.