Thorough Examination for pallet trucks, stackers and fork lift trucks is roughly the equivalent to the MOT for cars - a mandatory check to ensure that the mechanical parts of your lifting equipment are in safe working order. It is not the same as regular maintenance - so you need one whether or not you've just had the truck serviced.
Thorough Examination is required by law and must take place at regular intervals. That will depend on the type of truck you're running ... but it will certainly be at least once a year, often more. PUWER and LOLER regulations legally require all lift trucks to have a valid certificate of Thorough Examination. For peace of mind, you should always get your Thorough Examination from a CFTS-accredited competent person.