  The Event Horizon Telescope (EHT) has mapped the central compact radio source
of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution.
Here we consider the physical implications of the asymmetric ring seen in the
2017 EHT data. To this end, we construct a large library of models based on
general relativistic magnetohydrodynamic (GRMHD) simulations and synthetic
images produced by general relativistic ray tracing. We compare the observed
visibilities with this library and confirm that the asymmetric ring is
consistent with earlier predictions of strong gravitational lensing of
synchrotron emission from a hot plasma orbiting near the black hole event
horizon. The ring radius and ring asymmetry depend on black hole mass and spin,
respectively, and both are therefore expected to be stable when observed in
future EHT campaigns. Overall, the observed image is consistent with
expectations for the shadow of a spinning Kerr black hole as predicted by
general relativity. If the black hole spin and M87's large scale jet are
aligned, then the black hole spin vector is pointed away from Earth. Models in
our library of non-spinning black holes are inconsistent with the observations
as they do not produce sufficiently powerful jets. At the same time, in those
models that produce a sufficiently powerful jet, the latter is powered by
extraction of black hole spin energy through mechanisms akin to the
Blandford-Znajek process. We briefly consider alternatives to a black hole for
the central compact object. Analysis of existing EHT polarization data and data
taken simultaneously at other wavelengths will soon enable new tests of the
GRMHD models, as will future EHT campaigns at 230 and 345 GHz.
