What does this equation mean?

where $N_{206}$ and $N_{207}$ are the radiogenic lead atoms accumulated from each chain and $N_{238}$ , $N_{235}$ are the surviving parent uranium atoms. Plotting these two ratios against each other traces a curve, the concordia, on which a grain that has behaved as a closed system since crystallization must fall; a grain that has lost lead at some later date instead falls off the curve along a discordia line whose two intersections with concordia record both the original crystallization age and the age of the later disturbance [ 17 ] . This is the internal cross-check the directive of this article’s research promised: no external assumption is needed to catch a disturbed grain, because the two chains inside it disagree with each other the moment something has gone wrong, and the amount and direction of the disagreement diagnose what happened rather than merely flagging that something did.