This is a four-character idiom (成语) from ancient Chinese literature. The literal image is 'a clever tongue like a reed' (the reed in a musical instrument), describing someone who speaks with extreme eloquence and persuasiveness, but often in a misleading or deceptive way. It is almost always used with a negative or critical tone to imply that the speaker is being overly slick or manipulative. In modern usage, you can use it to accuse someone of being too smooth-talking and not trustworthy.