What is the meaning of "ITEM_ID_LIST"?

What is the meaning of "ITEM_ID_LIST"?

at the code below, what is the meaning of "ITEM_ID_LIST".
I don't know what to replace this..."ITEM_ID_LIST"
ArrayList skuList = new ArrayList();
skuList.add("premiumUpgrade");
skuList.add("gas");
Bundle querySkus = new Bundle();
querySkus.putStringArrayList("ITEM_ID_LIST", skuList);