Could I have an explanation in simple terms of adenocarcinoma of rectum with sacral matastatis? Thanks!

It is. Simply the most common form of colon and rectal cancer but in this case involves the rectum but has penetrated through the rectal wall and involved the sacrum or lower spine. This is most likely a direct extension of the cancer into the bone of the lower back. This is probably un-resectable for cure unless it is pretreated with neo-adjuvant radiation and chemotherapy before surgery.