taru-adjective, adverb taking the 'to' particle
lonely and dejected; alone and crestfallen
Literary yojijukugo describing a solitary figure in a state of deep sadness and discouragement. Often used in narrative or poetic contexts.
He wandered the streets alone and dejected, lost in despair.
He sat on the bench, a lonely and crestfallen figure.
孤独 is a common noun/adjective for 'loneliness' or 'solitude', while 孤影悄然 is a literary, emotionally charged yojijukugo emphasizing visible dejection.
悄然 alone means 'dejected' or 'dispirited', but 孤影悄然 adds the nuance of being alone and visibly forlorn.
A yojijukugo composed of 孤 (solitary), 影 (shadow/figure), 悄 (quiet/sad), and 然 (state). The exact historical origin is uncertain, but it is a classical literary expression for a lonely, downcast appearance.