What does this equation mean?

Uranium’s usefulness as a clock is doubled by an accident of nuclear structure: natural uranium consists of two isotopes, uranium-238 and uranium-235, which decay through separate chains to two different stable lead isotopes, lead-206 and lead-207 respectively, at two different, independently measured rates. Uranium-238’s half-life is about four point four seven billion years and uranium-235’s is about seven hundred four million years, values fixed by direct laboratory counting experiments [ 6 ] and formally adopted as the field’s working convention shortly after [ 7 ] . Because both chains run inside the same mineral grain simultaneously, from the same crystallization event, a single zircon crystal reports two ages that must agree if nothing has disturbed the system since: $\frac{N_{206}}{N_{238}} = e^{\lambda_{238} t} - 1, \qquad \frac{N_{207}}{N_{235}} = e^{\lambda_{235} t} - 1$. 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.