Aku is the gunk floating on top when you are cooking in liquid, whether in a dish like a hotpot or making stock. Aku is considered unsightly and clouds the flavor or your dish so you must remove it! 

The Aku-tori (scum remover is the unfortunate English translation) is a necessary item for any cook as it easily removes the aku without unnecessarily removing the broth with all the umami. 

Stainless steel

Product of Japan