What is zombie Coins?

Could you please elaborate on the concept of 'Zombie Coins'? I've heard the term but am not entirely sure what it refers to in the realm of cryptocurrency and finance. Are these coins somehow related to defunct or abandoned projects? Do they pose any risk to investors? Or is it simply a metaphorical term used to describe a certain phenomenon in the crypto world? I'm quite curious to know more about this intriguing aspect of the industry.