In this task participants must report the colour of the letters spelling each word, but letters themselves also spell a colour name and this may be the same or different to the colour of the letters. Stroop (1935) reports that reaction times are slower the letters spell a colour that is incongruent with the colour of the letters, indicating a compulsory automated reading of the word.
Stroop, J. R. 1935. Studies of interference in serial verbal reactions. Journal of Experimental Psychology 18: 643-662