A line is made of an infinite number of points that are right next to each other. A line goes on forever in both directions. In geometry, a line is always straight, so that if you know two points on a line, then you know where that line goes.
A line is only one point thick (which is to say that it has no thickness, because a point has no thickness). When we mark off certain points on a line, we can use that line as a number line. When we draw a line with a pencil, we make it thick enough to see. But a line in geometry is not thick enough to see, because it is only one point thick. If you looked at a line from the end, it would look just like a point – so it would be invisible.
If you took a line and moved it through space, like sliding a flat ruler across a table, it would create a flat plane. You can create half a line by choosing one point on a line and naming the section of line that goes on from that point.
You can also create line segments of any length by choosing two points on a line and naming the section between those two points. We use line segments for many things in geometry: one side of a triangle or a square is a line segment.