Those consequences, of course, are the punishment Adam will receive if he breaks the one law in Paradise. He will condemn him to death, though not immediately. This shows God’s omniscience, in that He knows that Death, son of the daughter of Satan, will soon reign over Earth. On the other hand, it also shows God’s compassion towards His newest creation, Man. If He had truly seen Adam and Eve’s disobedience as bad, He would have destroyed them on the spot. Instead, He chooses to let them live, though mortal, to continue to suffer in “a world of woe and sorrow.”
Furthermore, Satan himself, the tempter, knew that the consequences of eating the fruit would be worse over time. Even before his plan is fully laid out, he knows that his actions will result in worse sufferings for Adam and Eve. This all seems somewhat harsh, but remember that God gave plenty of forewarning about the consequences of their actions.
It's also important to note the adjective describing the consequence, specifically the word "bitter." For one thing, it references the previous line, as if the consequences of the action "taste" just as bad as the apple it itself. This isn't the first time that Milton uses the word "bitter," however. He mentions it in Book 4 (line 24) as well, this time in regards to the memories Satan has of Heaven, when he was an upper-ranking Angel. This is an important reference as well, because soon Adam will lose the glory of Eden.