Around the 13th century in Britain the church held most of the power. It possessed a huge amount of the land and wealth in Britain at that time. It also had its own court of law and it could try anyone it liked for committing any number of sins, heresies and the like. The person whose job it was to summon people into the ecclesiastical court was called the summoner.
Summoners were in a position where they could quite easily abuse their power, and more often than not they did. The summoner would often accept a cash payment or other bribe rather than taking the poor individual to court. The causes for this stem on the same beliefs that allowed the pardoners to go about their business - you could be freed of sins for a price. As they were reasonably high up members of the church hierarchy, they were free the bounds of normal law, the only laws applying to them were canon laws. This meant there was little people could do to stop or challenge the summoner. It did not take the summoners long to realise that they could order false summons on people and take the money from the bribe for themselves
Being of a similar disposition, summoners and pardoners would often go around in pairs, travelling from village to village. This is what the summoner and pardoner do in The Canterbury Tales, which contains one of the best descriptions of a summoner.