Lines in the plane are represented by a line segment with arrows on both ends. We can identify two lines in the diagram: one that goes through points R and O, and one that goes through points F and L.
We can use any two points on a line to name it. Point R and O are on the first line, so we can name it RO. The order of R and O doesn't matter and is, thus, an arbitrary choice. By the same principle, one name for the second line is FL.