What did jesus mean by saying, no one has ascended to heaven but He who came down from heaven,..... john 3:1?

we have heard of people who are in heaven, like, Enoch (Genesis 5:24), Elijah(2kings 2:11), Abraham and Lazarus. I need more light to what jesus said; one has entered into heaven except he who came down from heaven, that is to say, only him-jesus