。
Diligently turn off the lights in empty rooms.
Literal
People [subject-の] not-exist room [attributive-の] lighting [object-を] diligently turn-off.
An energy-saving tip or instruction. Two noteworthy features: (1) 人のいない shows the が → の substitution in relative clauses (the subject of the subordinate いない would normally be marked with が, but inside a noun-modifying clause の is used instead); and (2) こまめに is an adverb meaning 'diligently, frequently, thoroughly' — often used for small, regular actions done conscientiously.