To be a bit more precise, the Nankai ("South Sea") Trough is a tectonic subduction zone to the south of Japan, roughly the entire length from Izu to Kyushu:

https://en.wikipedia.org/wiki/Nankai_Trough

And Nankai megathrust earthquakes are those caused by plate movement there:

https://en.wikipedia.org/wiki/Nankai_megathrust_earthquakes

Statistically major earthquakes occur here every 90-200 years, and the last was in 1946.