Volcanic ash is a volcanic rock which is exploded from a vent in fragments less than 2mm in size.  Volcanic ash-particles are like small sharp glass-particles that damage anything they come across.  Because volcanic ash can fall on many things it's very harmful to the environment around the volcano.  During heavy ash-rains houses and buildings may collapse, people and animals may die by lack of oxygen.

     Ash clouds may provide big problems for aviation.  When airplanes fly through an eruption cloud a range of damage may occur depending on the concentration of volcanic ash, gas aerosols in the cloud and the actions taken by the pilot to exit the eruption cloud.

(Fig. 2.38) This pilot tries to clean of the dust off his plain during the eruption of the Vesuvius in March,1944(WW2).  He wears an helmet to protect himself for falling ash and pumice thrown out by the volcano.
Credit: USAF