Writing to learn involves giving students short, informal writing assignments that are used for learning and clarifying thoughts. We can use writing for authentic purposes that are synthesized with the learning process in all content areas.
This means that math teachers, chemistry teachers, art teachers, history teachers and even physical education teachers can engage students in authentic writing activities. Although writing is an important part of any English class, it needs to be used across the curriculum as well. For example, if you are a physical education teacher, you could have your students keep a fitness journal where they write their exercise plan or document their workout habits to keep track of their progress. A math teacher, for example, might ask their students to write down the procedures for doing long division, helping them process and check their understanding.
Writing is a part of our life and can help us learn and and clarify our thoughts. As the students use writing as a learning tool, both critical thinking skills and general writing competence can be developed.
This video should be helpful for your understanding of how some teachers use writing to learn. This website gives many examples of writing to learn tasks.
The following is a link to a file that is very informative on this topic.