Lit., '(with) light being absent, darkness prevails.' 'Luce absente' is an ablative absolute, a construction equivalent to many kinds of clauses, here to the protasis of a general condition, such as 'when(ever) light is absent.' Alternatively, one could express the protasis fully and say 'cum lux abest, obscuritas obtinet.' Other ways of expressing this are 'si lux abest, obscuritas obtinet' (a logical--also known as simple or real--condition), and 'luce indigente, tenebrae obtinent'.