At its very basic, a lesson plan is a structured course of instruction that a teacher uses to teach. A lesson plan can be broadly created for an entire subject, for a month, for a week, for a single class etc.
The idea behind a lesson plan is not to have a rigidly structured schedule whose success hinges on a set amount of learning being done on one day. The lesson plan is not EVEN for the teacher to follow a schedule. Rather, it is supposed to help the student. A well-developed lesson plan lays down an end objective, details how learning for that particular subject should happen, sets goals and milestones for students, makes allowances for students who need time to catch up, accommodates ones who have raced past all milestones and more importantly, it a lesson plan runs along the lines of what the teacher considers the philosophy of education.
A lesson plan does not have to impinge upon the spontaneity of teaching. Rather, it is a collection of parameters that allows you to judge at the end of a day, week, month or year how much learning has been carried out, which student needs more help, which student should be given some advanced material and it also tell you how many concepts have been learnt and how many more need to be learnt.
Lesson plans sound intimidating but actually, they are pretty simple. Attached here is a sample lesson plan that should give your a fair idea of how lesson plans work.