It refers to zazen that aims to reach "taigo" (also called "satori," which means "enlightenment") through emphasizing, digging into, and grasping "koan" (a story, question, problem, or statement generally inaccessible to rational understanding, yet which may be accessible to intuition).
公案を重視し、研究し理解することで大悟（悟り）に至ろうとする禅のこと。