The r-process (rapid neutron-capture process) is a nucleosynthesis process consisting of repeated neutron captures more frequent than beta decay (because many neutrons are being produced and/or beta decay is being suppressed), resulting in synthesis of isotopes with a mass number more than one higher while remaining with the same atomic number. When conditions change and beta decays occur freely, they repeat until the nucleus reaches a stable configuration, but the end elements/isotopes are not what they would otherwise be. The r-process explains the abundances of some elements, such as germanium, xenon, platinum and other elements heavier than iron and nickel.
The process is presumed to happen in core collapse supernovae due to models but observational evidence has only been indirect, via the success of such models in explaining other observable phenomena. The first observational evidence of spectra showing the characteristic radioactivity (beta decay from heavy elements) is from GW detection GW170817, a neutron star merger, a type termed a kilonova. The possibility of r-process occurring in kilonovas had already been theorized.
Neutron capture requires free neutrons, which have a limited lifetime (a mean lifetime of about 880 seconds). In events that produce such free neutrons, the ratio of neutrons to all nucleons at each point in space and time is of interest, typically cited as the electron fraction (Ye), the fraction of nucleons that are protons (because wherever there are protons, there must be a similar number of electrons in the general region, and vice versa, or else the region has a strong electric charge).
The spectral signatures of elements attributed to the r-process can be used to identify and age old stars. Very old (high redshift?) galaxies have been observed with r-process-produced elements but not s-process-produced elements, suggesting that at an early time, the r-process was working but the s-process was not. Individual stars with similar characteristics are presumed to be of similar age.