Frequencies table serves as an alternative to the default route > trip > stop_timings system in GTFS. Read the specs here.
Normally, we have to lay out each trip in the day that a route takes in trips table, and then expand each trip in stop_times table with the specific timings.
But for some transit systems, the route may be plying at a certain frequency, for example every 30 mins between 5am and 11pm. And even the journey may be the same throughout. (ie, trip sequences or time between stops don't change)
Instead of having to define each trip (72 times in the example case), the GTFS spec enables setting up a special "frequency" trip in the frequencies table.
The stop_times table defines the sequence of stops and the time difference between each stop.
Create a new frequency:
Note: A frequency-based trip_id defined here needs to have an entry in trips table too, and the sequence of stops in this trip needs to be defined in stop_times table.