The Lesson Goal tells you precisely what a student should be able to do after you have taught the lesson.
The goal should be described in terms of a performance, not a state of mind. Most lessons will have a single goal since it is difficult to learn more than one thing at once. The What to do section of the lesson should include an opportunity for students to demonstrate that they have achieved the goal, though this can sometimes be difficult to arrange in whole class teaching. If you are having trouble formulating the goal for a lesson you have created, it may point to a pedagogical issue with the lesson itself.