...are any hadron which is made of three quarks (qqq).
they are made of two up quarks and one down quark (uud), protons are baryons. So are neutrons (udd).
...contain one quark (q) and one antiquark ().
One example of a meson is a pion
which is made of an up quark and
a down anitiquark. The antiparticle of a meson just has its quark and antiquark switched, so an antipion (-)
is made of a down quark and an up antiquark.
Because a meson consists of a particle and an antiparticle, it is very unstable. The K meson lives much
longer than most mesons, which is why it was
called "strange" and gave this name to the strange quark, one of its components.