  There are two mass generating mechanisms in the standard model of particle
physics (SM). One is related to the Higgs boson and fairly well understood. The
other is embedded in quantum chromodynamics (QCD), the SM's strong interaction
piece; and although responsible for emergence of the roughly 1 GeV mass scale
that characterises the proton and hence all observable matter, the source and
impacts of this emergent hadronic mass (EHM) remain puzzling. As bound states
seeded by a valence-quark and -antiquark, pseudoscalar mesons present a simpler
problem in quantum field theory than that associated with the nucleon.
Consequently, there is a large array of robust predictions for pion and kaon
properties whose empirical validation will provide a clear window onto many
effects of both mass generating mechanisms and the constructive interference
between them. This has now become significant because new-era experimental
facilities, in operation, construction, or planning, are capable of conducting
such tests and thereby contributing greatly to resolving the puzzles of EHM.
These aspects of experiment, phenomenology, and theory, along with contemporary
successes and challenges, are sketched herein, simultaneously highlighting the
potential gains that can accrue from a coherent effort aimed at finally
reaching an understanding of the character and structure of Nature's
Nambu-Goldstone modes.
