Intumescent means tumour forming. What on earth would you want one of those for in DIY?
These are strips that go into grooves routed into the sides and top of a door. These are a common sight in large buildings.
They prevent fire spread from room to corridor, or vice versa, and protect occupants in other rooms from the smoke, which can be toxic.
Flammable surfaces in need of fire protection where no visible bulk would be acceptable can be painted with intuescent paint. It isnt cheap.
May be used to add fire resistance where materials are joined.