Reactive maintenance is fixing things after they break: the boiler dies, someone raises a ticket, an engineer attends. For cheap, non-critical assets it can be a rational strategy; replacing a kettle on failure is fine.
For anything critical, reactive-heavy maintenance is a warning sign. The industry rule of thumb is that unplanned work costs several times its planned equivalent once downtime and emergency call-out rates are counted. The ratio of planned to reactive work is a standard FM health metric.