An item analysis is a statistical method used to determine the quality of a test by looking at each individual item or question and determining if they are sound. It helps identify individual items or questions that are not good questions and whether or not they should be discarded, kept, or revised.
An item analysis is a post hoc test which means it is a measure used after the test has been taken. Many teachers use an item analysis after a test to be certain all of the questions are fair.
For example, a teacher runs an item analysis after her students latest exam and notices that all of the students missed question five. This suggests that question five is not acceptable for some reason because there should never be a question that all of the students missed. The teacher looks at the exam and realizes a question from the next section accidentally got formatted into the exam and the students have not been taught this concept. The question is then discarded and the teacher doesn't penalize the students for this bad question.