This chengyu literally means 'an empty cave invites the wind,' which originally described how rumors have some basis in fact. However, in modern usage it is very commonly used—incorrectly, from a classical perspective—to mean 'unfounded rumor' or 'baseless talk.' As a fixed expression (成语), you use it as a noun phrase or predicate to talk about rumors, often with the twist that the speaker may be unsure whether the rumor is true or false. Be aware of the ambiguity: most native speakers now use it to mean 'without evidence.'