The Runestone is a Mystical Viking artifact found on an island in the Nazi-occupied Danish Straits. Such runestones are dedicated by Vikings and other Scandinavian people to their gods in Asgard.
The Runestone was one of the relics studied by Johann Schmidt when HYDRA took control of a castle situated in one of the Danish Straits, during the German occupation of Denmark in World War II. Schmidt finished his study by the time Captain America and the Howling Commandos invaded the castle on their mission to destroy HYDRA facilities across Europe.
Schmidt sent an electrical surge through the batteries of the room containing the Runestone, that would cause an explosion that would destroy the facility and everyone inside. Schmidt offered Captain America to save himself using his enhanced speed, or to die with the rest of his unit.
Rogers prompted Dum Dum Dugan to take the Howling Commandos and escape, while Rogers would stay behind and save everyone, attacking the Runestone with his shield and cutting the cables attached to it. This move delayed the explosion enough to let the Howling Commandos and Captain America himself to escape unharmed.