The bag-drop (or check-in) cutoff is the latest time an airline will accept your checked bag before a flight - after that, you can still occasionally make the flight with cabin baggage only, but a checked bag will not be accepted.
It's set as minutes before the SCHEDULED departure time, not the actual one. If your flight is delayed, the cutoff doesn't move with it unless the airline formally retimes the flight in its systems.
This varies by airline. Full-service international carriers commonly use 60 minutes before departure for international flights, 45 minutes for domestic. Some low-cost carriers are stricter - a well-known example is Ryanair, which uses a 40-minute bag-drop cutoff and closes boarding 30 minutes before departure, both earlier than most full-service airlines.
Missing a bag-drop cutoff is one of the two "binding constraints" this app checks for every estimate (the other is gate close) - whichever is tighter for your specific flight and airline is what actually determines your recommended leave time.
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.