More precisely it's the interval in which the bandwidth will be throttled, i.e. every 200ms here.
The lower the number of milliseconds the smoother your bw graph will look, if you set it to a bigger interval it will be throttled less often and therefor the bw graph won't be as smooth, the bw will go up and down a bit around the throttling limit you set.
On the other hand throttling needs cpu time, the lower you set this number the higher your cpu usage will be.
Adjusting this is usually not necessary, the default is fine