阁下 is a formal honorific used in Chinese to address or refer to someone of high rank — equivalent to 'Your / His / Her Excellency' in English. You'll encounter it in diplomatic correspondence, court proceedings, or highly ceremonial spoken contexts (e.g., 总统阁下 'His Excellency the President'). It always follows the person's title/name (外交部長阁下) or stands alone as a direct address (尊敬的阁下). Never use 阁下 in casual conversation — that would sound stiff or even sarcastic.