It's a play on the word "can": to be able to do something; and the metal container. They eat what they can (ie. what they are able to eat)... ...and they (put in a) can what they can't (eat).

The word "thy" , if you have writen correct dictation, means the thing that belong to you and used when you talking to only one person. e.g. : Honour thy father and thy mother. Your sentence means : They eat whatever things that belong yours and they are able to do impossible things. Can also means demiss in informal way. تکلموا تعرفوا فان المرء مخبوء تحت لسانه

They eat what they can and they can what they can't. I think it either means what Serah said, or that it means that they eat as much as they can, and put the rest in the trash 'can'.

they eat everything that people can eat; nothing is impossible

中文意思： 饥不择食 无所不能