Frequency is the total amount of minutes to complete 1 line for a single bus/tram/train devided by the number of busses/trams/trains on that line.
Im pretty sure frequency is an average of that.
If this is true then a better way of explaining frequency is to say that its the average waiting time for stuff or passengers to wait for their train to arrive. This means that this does not include travel time, or time to get to the train/tram/bus station/stop.
Build a line with just 1 bus. Look at the frequency. Then build another bus right after it. Ull see that the frequency has halved, altho the busses are driving right behind eachother.
I hope you understand this haha. This is what i figured out, I could be wrong. But in that case, i have no idea what im doing, but i must be doing something right then.