It roughly means "which functions as". For example, if I said that I had a home office cum guest room, it means that I have a room that I use primarily as my office, but I also use as a guest room when I have overnight visitors.

It just means "together with." It is the Latin word "cum," as in "summa cum laude." It's not pronounced the same way as "come." The "u" is pronounced like the "oo" in "took." You see this less and less because it is spelled the same way as a vulgar slang word.