An MOT is a thorough annual examination of your vehicle. They are required when your vehicle is three years old or older. The main things that are covered by the MOT are safety of your vehicle and the level of emissions in the exhaust.
Cars & Vans must be examined every 12 months, and this is the owner's responsibility. Without an MOT certificate, you are unable to renew your road tax and your car may not be insured. It is a legal requirement to have a current MOT certificate in the UK.
Motoring safely is of course of paramount importance, so the checks undertaken are all geared towards identifying problems that need immediate attention, but you may also be advised of the need to have other aspects looked at in the future (advisory items).