It says in the Gemara of Shabbath, Ain Bishul Ahar Bishul - once an item has been cooked, it cannot be cooked again. However, there is cooking after baking or roasting. This means that an item that was baked in an oven can, subsequently, be cooked in a liquid.
As a result, we are not permitted to put bread into soup on Shabbath, or in any liquid that is hot to the extent of "Hayyad soledeth bo", which the Ben Ish Hai quantifies as too hot to eat or drink without waiting for it to cool down.
In fact, this does not apply only to bread, but this ruling has been extended to forbid placing any food in a liquid that is hot to the level of Yad Soledeth Bo.
(See Ben Ish Hai, shana beh, parashath Bo, oth heh and oth waw)
