What you just learned is called a "P-controller" where P stands for proportional.
Here is a really trick question by which I want to test your intuition--
one that doesn't have a unique answer, but it has a best answer.
Suppose you do what I just said. You steer in proportion to the crosstrack error.
That is, your steering angle is proportional by some factor of tau to the crosstrack error.
What will happen with the car?
It never quite reaches the reference trajectory? It overshoots?
Or either can happen?