Everybody's heard about the volcano, Eyjafjallajokul, in Iceland, erupting and causing a huge cloud of volcanic ash to rise above Britain and Europe. It stopped hundreds of flights, infuriated thousands of people who were planning to travel, and cost Britain's airports millions of pounds. Many people think that the reason so many airports were closed is because the pilots would not be able to see where they were going. This is not true. Pilots have compasses, airline technology and radius to show where they are and which direction to go. You could just say the windows are there to make the plane look pretty.
The reason it was a risk for aeroplanes to fly in these circumstances is because of the ash itself. Volcanic ash consists of tephra, or pyroclasts, which consist of pulverized rock and glass. On every aeroplane there are two jet engines, also known as gas turbines, which generate thrust - the wide, circular things under the plane's wings, consisting of a fan, a turbine, a compressor and other bits and bobs. The purpose of this jet engine is to move the aeroplane forward at great speed, on and off the ground.
These jet engines are wide open at the front - plenty of air gets into the engine and around the engine's core. Some of the air is made hot, and some of it is made cold. The hot and cold air is then mixed together at the engine exit area, making thrust and pushing the plane forward.
If a plane flew into the volcanic ash cloud, the dirty air full of rock and broken glass would get into the jet engines along with normal air. It is not a definite consequence, but there is a high risk that if volcanic ash got into the jet engines, the volcanix ash would damage the plane, and the plane would literally fall out of the sky.
There have been a few incidents where planes have flown over or into volcanic ash clouds, once when British Airways flew over Indonesia in 1982 and had to make an emergency landing because of failed engines, and once when KLM flew over an ash cloud from a volcano in Alaska and had to land early. The British and European airports clearly don't want any incidents like this happening again. Thousands of angry alive people is better than thousands of dead people, damaged aircraft, grieving families and sue cases.
You could say that airports are taking Health and Safety to a ridiculous level, but many have said that they would rather be safe on the ground and miss their flight than be up in the air in danger. If the airports hadn't closed down, someone you know could have been in danger when going on holiday or on a business trip, and possibly could have died. Better safe than sorry.