The Catechism (# 1023) states that faithful who die after receiving Baptism "… already before they take up their bodies again .... will be in heaven ...." Could you please explain and elaborate on just what this means?

Currently, prior to the Second Coming, when we die our bodies lie in the earth, but our soul goes to God. Those deemed worthy and capable of heaven, after any necessary purification, are admitted into heaven.
 
Only at the Second Coming, will our bodies rise, when, as Scripture attests, the trumpet shall sound and the bodies of the dead will come forth (1 Cor 15:52). Our body which rises, will be truly our body, but gloriously transformed as St. Paul details in the same place in Scripture (1 Cor 15:35ff)
 
What this most fundamentally means is that Christ did not come to save only our souls but the whole of us, soul and body. One great dignity of the human person is that we unite both the spiritual and material aspects of God’s creation in our very person. This glory will be restored to us at the Second Coming.
 
An additional meaning of this truth is that we must reverence both our soul and body through the spiritual and corporal works of mercy and we must fulfill the mandate to glorify God in our body as well as our soul.