A lesson plan is a teacher's plan for teaching a lesson. It can exist in the teacher's mind, on the back of an envelope, or on one or more beautifully formatted sheets of A4 paper. Its purpose is to outline the "programme" for a single lesson. That's why it's called a lesson plan. It helps the teacher in both planning and executing the lesson. And it helps the students, unbeknownst to them, by ensuring that they receive an actual lesson with a beginning, a middle and an end, that aims to help them learn some specific thing that they didn't know at the beginning of the lesson (or practise and make progress in that specific thing).