Aiming at massive participation and open access education, Massive Open Online Courses (MOOCs) have attracted millions of learners over the past few years. However, the high dropout rate of learners is considered to be one of the most crucial factors that may hinder the development of MOOCs. To tackle this problem, statistical models have been developed to predict dropout behavior based on learner activity logs. Although predictive models can foresee the dropout behavior, it is still difficult for users to understand the reasons behind the predicted results and further design interventions to prevent dropout. In addition, with a better understanding of dropout, researchers in the area of predictive modeling in turn can improve the models. In this paper, we introduce DropoutSeer, a visual analytics system which not only helps instructors and education experts understand the reasons for dropout, but also allows researchers to identify crucial features which can further improve the performance of the models. Both the heterogeneous data extracted from three different kinds of learner activity logs (i.e., clickstream, forum posts and assignment records) and the predicted results are visualized in the proposed system. Case studies and expert interviews have been conducted to demonstrate the usefulness and effectiveness of DropoutSeer.