What does this equation mean?

The relation itself is old and simple. For a spherical, non-rotating exterior, the surface redshift $z_s$ follows from the mass M and radius R as $z_s = \left(1 - \frac{2GM}{Rc^2}\right)^{-1/2} - 1$. — a function of the star’s compactness alone, so that whatever fraction of a percent of uncertainty NICER puts on M and R propagates directly into the redshift. The paper works through six NICER-mapped pulsars this way, and two of them mark the range worth remembering. PSR J0740+6620, the most massive precisely-timed neutron star known, comes in at roughly 2.07\,$M_\odot$ with a radio-timing mass fixed independently by Shapiro delay to 2.08$\pm$0.07\,$M_\odot$ [ 4 ] , giving a compactness near 0.25 and a surface redshift $z_s$$\simeq$0.40 — the deepest gravitational potential in the sample, though also, because its radius is the harder-measured half of the pair, the least precisely known redshift, at roughly fifteen to sixteen percent. PSR J0437-4715 sits at the opposite end: the nearest and brightest rotation-powered millisecond pulsar NICER has targeted, with a radio-timing mass prior tight enough to pin the pair at M=1.418$\pm$0.037\,$M_\odot$ and R=11.36^{+0.95}_{-0.63} km [ 3 ] , which maps to $z_s$=0.257$\pm$0.026 — a ten-percent redshift standard, the tightest of the six, built entirely from timing and pulse-shape data with no spectroscopy anywhere in the derivation.