It's simply knowing Christ after the flesh. This is where many Christians spend most of their spiritual life. And yada in the Old Testament often refers to the marriage act. It's interesting because we can be married to someone and even have sex with them for years, and yet never actually experience that intimacy that God designed for each of us to have. The physical act alone is not what gives us intimacy.
Intimacy occurs when our hearts, souls and bodies become one.
And it's the same thing with our relationship with Jesus. We can know Him as Savior and yet still be totally consumed with ourselves and thus conformed to this world. God says the result of this way of thinking is captivity, imprisonment, and total bondage to the flesh. These precious people see things in their lives going just the opposite direction from what they hoped would happen, and rather than trust more as God would have them do, they pull in, get bitter and blame God. Fear and doubt then cover their hearts, quench God's Life and, as a result, they never go on to experience intimacy with God.