Collaborative learning has been advocated for the last several decades. However, classroom practice has indicated that this method often comes with challenges and low efficiency, suggesting the need for better support for teachers. The aim of this study is to explore teachers' learning about collaborative learning in a professional development (PD) environment. In particular, we focused on teacher perceptions of the two aspects of collaborative learning, namely, tasks and collaboration. Teachers learned that tasks for collaborative learning should be open-ended, challenging, and feature a low floor but a high ceiling. Such tasks call for true collaboration among group members and provide a space for interactive engagement. In addition, teachers learned that true collaboration should occur in two layers, including both peer collaboration and collaboration between the PD provider (researcher) and teachers. For peer collaboration, there should be 'deep participation' in which every group member actively contributes ideas and engages with others' thinking. For the researcher-teacher collaboration, the PD provider serves as the teachers' learning partner through careful task design and facilitation of learning. Based on their direct experience with collaborative learning in this PD workshop, teachers learned about the roles of tasks and collaboration in their own classrooms. Implications of the findings and future directions for research and practice are discussed.