He went to collect the souls who had been waiting (Limbo sort of place) for the Redeemer to open heaven for them.
annalex posted a wonder painting of this the other day — was it Good Friday. Check out the Daily Readings thread for that day.
This Orthodox icon,-- used in Catholic churches as well, but with lesser emphasis, -- explains the paschal mystery from the perspective of Holy Saturday when Christ defeats death, breaks the gates of hell (see Matthew 16:18), descends to Hades and liberates the souls of the Old Testament righteous that were awaiting His coming (1 Peter 3:19, compare Luke 4:19):
You see Christ surrounded by the oreol of glory standing on the broken gates of Hell and reaching to Adam and Eve, -- and in their persons to all of us. The hardware from teh gate is scattered underneath. The kings and prophets of the Old Testament form a welcoming party: we recognize King Solomon and King David by their crowns and Abel by his shepherd's staff.
His Cross is sometimes shown in the background; sometimes Satan is shown bound underneath.
The icon is known as Descent to Limbo, or Descent to Hades, or simply (as in this case) The Resurrection of our Lord.