The Lookback field is related to automated referrals. Automated referrals are those made when a user shares their invite / referral link on their Facebook page. If another user clicks that, signs up, and then visits, that counts as an automated referral (since that doesn't require the referral code). The Lookback feature allows you to check back x amount of days for any automated referrals for the guest within that time period.
If further assistance is required, submit a ticket to Punchh Support. (For help submitting a ticket, click here)