A lease clause giving the tenant the right to lease additional space should it come available.  The landlord is typically required to first provide written notice to the tenant, informing it of the availability of the space.  Tenant may then exercise its right before any other party is offered an opportunity to lease the space.  This clause is also sometimes offered in the context of a sale of the property, giving the tenant the first right to purchase the property should the landlord decide to sell.