Do you think Christ is just lying in the tomb today waiting for tomorrow morning? Oh, no! He has descended to the dead in order to preach to those in prison and liberate captives!
It is customary to represent Christ in paintings on walls, as He descends, appears before hell, clad in a priestly robe and with a banner in His hand, with which He beats the devil and puts him to flight, takes hell by storm, and rescues those that are His. Thus it was also acted the night before Easter as a play for children . . . . He descends with the banner, shattering and destroying the gates of hell; and we should put aside thoughts that are too deep and incomprehensible for us.
-- Martin Luther (1533)
You can read a fascinating paper on this called "The Harrowing of Hell: Filling in the Blanks" by The Rev. Peter Burfeind right here. It is from a Lutheran point of view.