The direct translation of this sentence into modern Chinese is: 'Longing is a kind of emotion that is inherently difficult to put into words.But the complete poem is “相思本是无凭语，莫向花笺费泪行”So the full meaning of the poem is: 'Longing is a kind of emotion that’s inherently hard to describe in words, so don’t just sit there crying over a letter all by yourself.