Sorry, a language deficit.
"Criterion" here is just a way of saying "whatever you want the criteria to be".
The criterion (single) or criteria (plural) is the set of Rules by which to filter, so if you only want to SELECT for times where hour1 is later than February 1, 2020, for instance, then the "Criterion" becomes "hours1 > '2020/02/01'" and the full statement becomes:
```
SELECT julianday(replace(hours2,'/','-')) - julianday(replace(hours1,'/','-'))
FROM mytable
WHERE hours1 > '2020/02/01';
```
But since we do not know what criteria you want to filter by, and it is irrelevant to the answer itself, we just added the word "Criterion" to complete the structure of the SELECT statement. It's up to you to decide what that is.