望眼欲穿 is a four-character idiom (成语) that literally means 'to stare so hard your eyes are about to pierce through' — it describes an intense, desperate longing for someone or something to arrive, often after a long wait. It's used in emotional contexts like waiting for a loved one's return or a long-awaited letter. Unlike the simpler 期待 (to look forward to), this idiom carries a sense of anxiety and physical strain.