To write an essay on the Stroop effect, you first need to understand and define it. The effect is named after John Ridley Stroop, the psychologist who published a paper on it in the Journal of Experimental Psychology in 1935. It deals with the phenomenon of cognitive interference caused by conflicting stimuli and resulting in a delay in processing.
The most widespread and perhaps the simplest demonstration of the Stroop effect ("the Stroop test") consists of writing or printing the words for colors in different colored inks so that, for instance, the word "green" is written in red ink, the word "yellow" in purple ink, and so forth. Stroop test studies show that people take longer to name the color and do so with a lower rate of accuracy than they do in tests where the name of the color is written in ink of the same color. This is because the stimulus of the word and the stimulus of the color conflict.
Depending on the instructions you have been given, there are a number of different applications of the Stroop effect which you might examine in your essay. You could look at the way in which it has been used in formulating theories of cognitive development by developmental psychologists such as Andreas Demetriou. Alternatively, since the classical Stroop test was developed, variations such as the emotional, numerical, and spatial Stroop tests have used the effect in different ways. Or you might examine attempts to integrate the Stroop effect into tests by employers to determine the extent and speed of an individual's processing abilities.