GMAT® is a question-wise adaptive test – GMAT® adapts itself to your performance while you are taking the test. While the test typically starts with a medium difficulty question, the question difficulty level changes based on whether you are getting questions right or wrong. If you are getting questions right, the question difficulty level rises and you start seeing more difficult questions. Likewise, if you are getting questions wrong, you start seeing easier questions. Based on your performance in the test section at different difficulty levels, your score is calculated.
Since the next question is generated based on the difficulty level of the current question, and your performance in the section till that point in time, you also cannot browse back and forth between questions.
Also, you MUST select an answer to a question (take a calculated guess if you are unsure of the correct answer) in order to proceed to the next question