This type of session is
the format of many of the types of training
explained elsewhere.
Fartleks, most speed work, pyramid and paarlauf sessions are all types
on interval session.
It is important to understand the term interval training
here as it is a term that, as an athlete,
you will often hear and if you go to a track, almost every runner will be doing interval repetitions.
Generally during an interval session "overloading" occurs where levels of oxygen debt and therefore
lactate / lactic acid levels are increased to a level that would be unsustainable were the whole session run as a single repetition.
The stress on the body creates adaptation after the sessions, so that the body is better able to cope with the demands placed on
it in the future - and hence levels of fitness and performance are increased.