The email will contain the signer's name and information about the packet. Your custom text will be between their name and the “Review and Sign” button.
Note that this message will only show in any emailed signature requests. With Workflows, if the primary signer also completes the last webform in the submission, they will proceed straight to signatures within the browser window. To customize any pre-signature messages within a Workflow, edit the “Review and Sign” page.