There's a limit on how many people can run the Boston Marathon.
To qualify to "run Boston", you have to run another marathon in a qualifying time[1], prior to applying. For example, the qualifying time for a male 40-44 is 3h05m. For a female of the same age, 3h35. Non-binary, 3h35.
You submit your application and qualifying race and time, and then some time _later_, based on the number applications received that are within the cutoff (and it's always more than they can accept), they adjust the cutoff time downwards even further. That additional cutoff delta is the what's being calculated on the slider here. So if your published cutoff is 3h05, and the slider predicts a 6min delta, you need to have run 2h59, not 3h05.