Chemical delignification is a process that breaks down the lignin in timber. Lignin is an organic polymer that acts as a glue in timber (amongst other things), giving structural integrity to cell walls and thus holding the fibres together. It's the same material that gets attacked by fungal decay. The result is similar, the timber loses its structural integrity and slowly flakes apart.
Delignification is often found in roof timber battens as depicted in these pictures, being caused by chemicals in the air. Often it is found near heavy industry, major roads or not far from the Sea.
If there is significant damage caused by delignification then it's best to get it checked for structural integrity by a builder, seeing that roof battens are often affected by this phenomenon, caution should be exercised when walking around on roofs.
See a video showing the obvious effects of chemical defibration on roof battens.