You should know the course material. If students are required to attend lectures and read assignments, then it seems reasonable that you would do the same.
Effective teachers can explain complex ideas in simple ways. As you develop expertise in an academic field, it is easy to forget that students may have no prior knowledge of fundamental concepts that you take for granted.
Keep your students thinking. Unless they are actively using the concepts you are teaching, most students will remember only a small fraction of what you teach