This is a vacuum-sealed pack of sliced rice cake, which is used in rice cake soup. You can find this in the frozen section of the Korean grocery store.
I used to make a huge amount of sliced rice cake by myself. What I did was soak short grain rice overnight and then take it to my local mill. They had a machine to steam the rice and make a large tube of rice cake out of it. I would bring this tube home and let it sit for a day so it would get solid. Then I took it back to the local mill and they sliced it for me. If I tried to cut it myself it was too much work.
These packages are more convenient. The taste is the same, the cakes are more uniform in size, no more blisters from slicing!