  The first combined detection of gravitational waves and electromagnetic
signals from a binary neutron star (BNS) merger in August 2017 (event named
GW170817) represents a major landmark for the ongoing investigation on these
extraordinary systems. In this short review, we introduce BNS mergers as events
of the utmost importance for astrophysics and fundamental physics and discuss
the main discoveries enabled by this first multimessenger observation, which
include compelling evidence that such mergers produce a copious amount of heavy
r-process elements and can power short gamma-ray bursts. We further discuss key
open questions left behind on this event and BNS mergers in general, focussing
the attention on the current status and limitations of theoretical models and
numerical simulations.
