Let's look at this picture. It's in one-point perspective. All of the lines that are parallel to us - the railway sleepers and fence posts - go straight across or straight up and down, and if they were longer, they'd keep going straight across, or straight up and down, staying the same distance apart and not meeting. The lines at right-angles to us, the ones moving away from us, come together at a vanishing point in the middle of the picture.
To draw one-point perspective, we arrange our subject so that one set of visible lines has a vanishing point right in front of us, and the set at right-angles goes out to infinity on each side. So if it's a road, it goes straight away from us, or if it is a house, one wall goes straight across in front of us, not sloping. In reality of course, there are always objects which won't be lined up perfectly, but for now, let's keep things simple!
