It's an equation to get an approximate value of the total ground take-off distance (including the distance after reaching Vr when it begins to rotate, until all wheels stop touching the ground). This is the equation that is on some lecture notes I found on the Internet:
It also specifies that in this equation, drag (D) and Lift (L) are not calculated with the maxmimum lift coefficient with surfaces in take-off configuration, but with a "typical lift coefficient" of 0.1. Drag (D) is calculated through the drag polar.