Where do you find this?
Asbestos roofing felt was often used for garage roofs, outbuildings etc.
What does this look like?
How dangerous is this?
Work on any type of asbestos can be dangerous. However, tasks on these particular materials can be carried out by non-licensed workers who are appropriately trained. Work on roofing felt would not normally be notifiable provided that the material is in good condition.
source: Roofing felt