This is the general dry-rot that is found on most roofs.  In this situation, the wood has rotted completely through the wood board and is visible from below.

Here you can see the dry-rot that has started on the top of the board.  Although the wood did not rot completely through, it has to be changed as the nails will not hold during the installation of the edge metal and shingles.  If the rotten wood is left, the rot would continue as it is a fungus and would have to be changed at a later date.

Corners are generally some of the worst areas where dry-rot can occur.  As most older roofs do not have edge metal, water can easily enter under the material and rot out the wood.

The light pieces of wood are the new ones replacing the rotted wood and in some cases boards that become broken due to age.