Depends on the terms of the options, but it means purchase rights to come back and buy aircraft at a predetermined price point and delivery date. Delivery date is the most important factor at the moment.
As the 787 is in demand at the moment, companies are exercising their options and getting their confirmed slots in the delivery queue. Letting an option expire in this seller's market might mean if you want a follow-on order you would be waiting for some years at the back of the line.