A teaching philosophy statement is a brief description of how you conceptualize teaching (and learning), including your beliefs, values, and practices. Your teaching philosophy, as a reflection of how you think about your own teaching, will likely change over time. It is something that you can add to a teaching portfolio, in a promotion and tenure packet, as part of a job application, or in your course syllabi.
There are many helpful guides for creating your unique teaching philosophy. Many centers for teaching at universities will include useful resources. Below are a few examples, but a Google search will produce many helpful results as well.