For the current value, it would mean 2mm of rain in 1 hour. For the forecast, it is 2mm of rain in 3h. For the daily forecast (not available with free API), it is 2mm for the day.
You can persist data every change or every update. When using averageSince, you need to take this into account.
If you are using a persistence strategy to save the current 1 hour rain volume data every 15 minutes, then you are persisting mm/h every 15 minutes. So, you will have 4 values per hour and the sum of the persisted values over the last 24 hours would need to be divided by 4 to get the 1 day sum. The average over the last 24 hours will provide the 15 minute average, so you will need to multiply by 96 (24 * 4) to estimate the 1 day average. Things would be more straightforward if you persisted every 1 hour.
For the 3h forecasts, you won’t be able to use persistence, but you can use this, which puts the 3h forecasts into groups to aggregate the values…