what what does George Will mean when he says independent candidates could Splinter the electorate why would this be important

It can be important since it makes a big difference in a close race, and a mainstream candidate loses (or wins) when the majority normally would have elected the other one.

"Splitting the Vote" implies that an outsider applicant comes in and strips away votes from some hopeful of the real gatherings. It can have a major effect in a nearby race, and a standard hopeful loses (or wins) when the greater part ordinarily would have chosen the other one.