Measurement line is an optional horizontal line you can use to force FontLab to calculate sidebearings at a particular position, instead of from the farthest extremes of the glyph.
Use View > Measurement Line (or the CtrlAltL keyboard shortcut) to show or hide the measurement line.
By default, the sidebearing values are measured from the rightmost and leftmost points of the glyph. But sometimes, you may prefer to measure sidebearing values from some other point on the glyph outline (perhaps you want to avoid including serifs, or take the midpoint of an outer diagonal, or….). In FontLab, the measurement line defines the “height” at which FontLab measures the sidebearings, instead of using the right and left extrema.