Servicing things before they break, so they're less likely to fail at all.
Preventive maintenance is distinct from maintenance in that it's done before any problem is evident. Rather than oiling your chain only when it's squeaky, preventive maintenance means oiling it regularly. The is more common with commuting and touring cyclists, who often have daily and weekly maintenance checklists (usually in their head). Check the brakes, lights and speedo every time they get on the bike, sit down every weekend and check the tyre pressure, oil the chain, adjust the gears, check the brakes.