1. to listen attentively; to pay attention to what someone says
Usage: often used with 'to'; somewhat formal or literary
2. to go back in time or memory; to recall or refer to something past
Usage: often used with 'back to'; less common than the listening sense

Examples

• Harken to the wisdom of your elders.
• The teacher asked the students to harken to the instructions carefully.
• If you harken back to the 1960s, you'll remember how different music was.
• The old song harks back to a simpler time.
• She refused to harken to his complaints.
• Harken! Do you hear that sound in the distance?