In order to enforce the terms of a lease a ground rent must be set, but in the past many leases had tiny ground rents so in some cases freeholders stipulated that the rent should, instead of money, be a ‘peppercorn’ (as used in pepper grinders) to save them the trouble of collecting the money. In theory the freeholder could still demand the peppercorn but in effect it means there's no ground rent to pay.
Ground rent is still charged by landlords, and is a condition of most leases. Ground rent is usually a small amount, typically £50 to £300 a year. If the amount is very small, or notional, it may be described as a ‘peppercorn rent’.