Merge fields are fields that you can use in emails that will be replaced with content. For example, you can use merge fields to insert a contact's name into your email's greeting. This way, even if you send an email to a whole list of contacts, each email will be individualized.
Big River provides many merge fields that you can use to get specific content into your email. Check out the FAQ on "What merge fields are available for use in emails?" to learn more.