The plotters rented a cellar under the Houses of Parliament and packed it with gunpowder. They planned to create a massive explosion when the King was in the Houses of Parliament. An anonymous letter was sent to the authorities warning them of the plot. This made them search under the Houses of Parliament and Guy Fawkes was caught guarding the gunpowder.
He was tortured and executed and ever since we have celebrated the failure of this plot on the 5th of November.
For a fun video explanation, please see below.