Each item in a test has its own ICC and the ICC is the basic building block of IRT. The steepness of the graph shows how well the item can differentiate examinees with low and high abilities. A flat curve is a poor indicator while a steep curve, like the one shown above, is a good indicator. If several ICC's are plotted in the same graph for the corresponding test items with the same shape, the curves on the left (or top) correspond to the easier items than those on the right (or bottom).
By analyzing the examinees' scores of each item from an exam using IRT software, one can have an idea 1) which questions are good indicators of assessing student abilities or not, and 2) objectively respond to which questions are "easy" or "hard".
Baker, F. The Basics of Item Response Theory. Available online here.
Graph is taken from http://echo.edres.org:8080/irt/ where one can also find a great deal of information on IRT.
Hambleton, R., Swaminathan, H., Rogers, H. Fundamentals of Item Response Theory. Newbury Park: Sage Publications. 1991.