The head gasket on a vehicles engine creates a seal to allow the engine to run properly and maintain the pressure it needs.
When the head gasket is blown then the engine doesn't get the proper pressure it needs to run properly and if you continue to drive the vehicle with a blown head gasket then you risk damaging the engine eventually beyond repair and you risk breaking down and being stranded on the highway.
A blown head gasket can be fixed if you get the head gasket replaced before the engine gets too much damage.