When referencing running, a split is a time from the first part of the race to the halfway point against the second. Literally, it is like splitting the race right down the middle. Your split time when you are training will help guide you to understand what your performance would ideally be at the halfway point of your race.
It helps you to know, while you are actively running if you need to keep your current pace or speed it up for the time you are working towards.