A radio-quiet neutron star is a neutron star that does not seem to emit radio emissions like most other neutron stars.
Most neutron stars are pulsars, and emit radio radiation.
Quark stars, theoretical neutron star-like objects composed of quark matter, may be radio quiet, according to some theories.
More plausibly however, radio-quiet neutron stars may simply be pulsars which do not pulse in our direction. As pulsars spin, they emit radiation from their magnetic poles. When the magnetic poles do not lie on the axis of rotation, and cross the line of sight of the observer, one can detect radio emission emitted near the star's magnetic poles. Due to the star's rotation this radiation appears to pulse, colloquially called the "lighthouse effect".
List of radio-quiet neutron stars