Most airlines and airports recommend arriving 3 hours before an international departure. It's a reasonable default, and it's what most check-in/bag-drop cutoffs (see below) are built around.
3 hours covers a typical day at a typical airport. It doesn't account for which airport, what time of day you're flying, whether security is backed up because several wide-body flights are boarding in the same window, or whether you're checking bags.
That's the actual gap this app fills: instead of one fixed number, it adds up your real segments - transport to the airport, check-in, security, immigration where it applies, and the walk to your gate - each as a range, and shows you a few different confidence levels for this specific flight.
Large hub airports at peak hours can run well past 3 hours end to end once you include getting to the airport itself - see the individual airport guides for real typical/busy-day numbers.
These are general, statistically-modelled estimates - not a guarantee. Always allow extra time for anything unusual, and check your airline's and airport's own current guidance.