@galaxypoint
Hello, the layer shows a number of lightnings in the given area in 1 square kilometer for the selected time, if you are not Premium user, the time step is 3 hours, for Premium users it is 1 hour.
Regarding the data calculation, you can read more about it on ECMWF website - https://www.ecmwf.int/en/forecasts/datasets/catalogue-ecmwf-real-time-products