This begins the polyline at PT1. Note that a 3d point need not necessarily be thought of as one thing. It's true that the list storing the coordinate values is a single object, but it is a complex object, itself consisting of a list of three elements. Thinking about it in this way, it's easy to conceive of a point as separate X, Y, and Z values combined together in some arbitrary but very useful association. CAR and CDR each take one required argument, which should be a list.
CAR returns the first element of a list.
1.0 CAR returns the first element of PT, which is its X coordinate value. At first glance, this doesn't appear particularly useful. When CAR receives this list as its argument, it returns the first element, which is the value of the Y coordinate of the point. If PT1 and PT2 hold the values of two points that form opposing corners of a rectangle, then the other corner points can be determined by combining the X and Y values of PT1 with the Y and X values of PT2. In our case, we ignore the Z values because we're dealing with a 2d polyline.