Pilot studies are small-scale practice investigations, carried out prior to research to identify potential problems with the design or method.
For example, participants may not understand the instructions or they my guess what an experiment is about. They may also get bored because there are too many tasks or too many questions.
If a researcher tries out the study using a few typical participants they can see what needs to be adjusted without having invested a large amount of time and money in a full-scale study.
The results of a pilot study are irrelevant – the researcher is not interested in the results produced – the researcher is simply seeing to what extent the procedures need fine tuning.