NANOGrav (North American Nanohertz Observatory for Gravitational Waves) is a pulsar timing array-based effort, using Arecibo and the GBT. It is an effort to detect gravitational waves through comparison of the timing of observed pulsar cycles. It currently (2018) observes about 50 millisecond pulsars.
A one nanohertz cycle has a period of about 30 years, for a the wavelength around 30 light-years or 10 parsecs. Likely, the title is meant to imply "a few nanohertz", i.e., a period of a few years. The NANOGrav effort began around 2010 and years of data are required to distinguish waves with such a long period (in that, it is something like determining lengthy extra-solar planet orbits from ongoing observations).
Similar efforts are the Parkes Pulsar Timing Array (PPTA) effort in Australia and the European Pulsar Timing Array (EPTA) effort. All the efforts collaborate and coordinate as the International Pulsar Timing Array (IPTA).