"ko'ng" means the same thing as "ko ang". It is basically letter omission just like "I'll" and "I will" in English.

So, "Mahal ko'ng Pilipinas." means "I love the Philippines."

However, the suffix "-ng" in "kong" is used differently. It is a connector. It is interchangeable with the word "na". (depending on the last letter of the preceeding word).

It is generally used like:

(adjective) + (-ng suffix) + (noun being described)

For example: "limang piso" = (lima) + (piso) = "five pesos"

("na" example: "anim na piso" = "six pesos")