Later in the course, things will get more complicated re degrees of freedom.
Let me try to give a short explanation of degrees of freedom for this part of the course.
Suppose we have a sample of five weights of puppies. To the nearest pound – we have a cruddy scale – they are 3, 4, 4, 5, ?. If I tell you the total of the five weights is 22 pounds, and then tell you the first four weights (3, 4, 4, 5), can you find the 5th weight? You should say “Sure!” Just add up the four weights – 16 pounds – and subtract from the overall total – 22 pounds – to find the missing weight is 6 pounds.
Once you know the four of the five, the fifth weight is not free to vary. That sample of 5 puppy weights has just 4 degrees of freedom, (n-1).
Although the math gets tricky in later parts of the course (after the midterm), the concept of degrees of freedom is similar to this.