6.9 is an intensity on a Japan Meteorological Agency Seismic Intensity Scale, which can vary from 0 to 7. So it's the second strongest intensity possible on the scale.
https://en.wikipedia.org/wiki/Japan_Meteorological_Agency_se...
https://earthquake.usgs.gov/earthquakes/eventpage/us6000t7zq...
This earthquake was measured as a 6.9 moment magnitude, sometimes referred to incorrectly as the "Richter" scale.
The Shindo intensity system is measured without decimal places. In this particular case, the intensity was a 6+. Which isn't the highest, but is still quite severe.
So an earthquake can be a 4 at the epicenter, a 3 a little further away, a 2 even further away, and so on.
Common online conversation with a friend in a different prefecture may be something like, "Last night's earthquake was a 3 here, how about you?"