What does this equation mean?

The uranium-lead and argon-argon methods both need to know, or be able to ignore, how much daughter isotope was already present when the mineral formed. The isochron method, used most often with the rubidium-strontium and samarium-neodymium systems, solves this by measuring several minerals from the same rock, each of which crystallized from the same well-mixed magma at the same time and so began with the same strontium or neodymium isotopic composition, but which incorporated different amounts of the radioactive parent isotope depending on their individual chemistry. Plotting each mineral’s present-day ratio of radiogenic daughter to stable reference isotope against its ratio of parent to reference isotope produces a straight line whose slope gives the age and whose intercept gives the initial composition directly: $R_t = R_0 + \frac{N_{87}}{N_{86}}\left(e^{\lambda t} - 1\right)$. where $R_t$ is each mineral’s measured present-day strontium-87 to strontium-86 ratio, $R_0$ is the shared initial ratio recovered as the line’s intercept, and $N_{87}$/$N_{86}$ is each mineral’s measured rubidium-87 to strontium-86 ratio [ 18 ] . The initial composition is no longer an assumption fed into the calculation; it is an output the data themselves supply, and a poor fit to a straight line is itself a warning that the minerals did not, in fact, share a common origin or have remained closed systems since.