Different inertial grids have different rel-coordinates for events. Here, two grids, Grid 1 and Grid 2, differ by a boost, in which Grid 2 is moving along the x-axis of Grid 1, at a speed β, with -1<β<1. The sign of β indicates if Grid 2 moves in the +x direction, or the -x direction.
Given the rel-coordinates of an event as measured in Grid 1, you use the boost transformation to find the rel-coordinates for the same event, as measured in Grid 2.
The units used in this chart aren't specified. It shows values only within a certain range, about +/- 225 or so.