MTF (Modulation Transfer Function) is one of the measurements that evaluates a lens' performance, and its contrast sensitivity at different spatial frequencies.
The horizontal axis is in millimeters and shows the distance from the center of the image toward the edges, and contrast value (highest value is 1) is shown in the vertical axis.

The readings at 10 lines per millimeter measure the lens's contrast ability (red lines), repeating fine parallel lines spaced at 30 lines per millimeter measure the lens' sharpness ability (green lines), when the aperture is wide open.
Fine repeating line sets are created parallel to a diagonal line running from corner to corner of the frame, are called Sagittal lines (S) and sets of repeating lines vertical to these lines are drawn, called Meridional (M) line sets.