What is the incumbency effect?

That a sitting president or the VP is more likely to win reelection to a 2nd term rather than a different candidate of the same party.

The incumbency effect is the advantage that elected representatives have over newcomers in getting re-elected. Voters usually prefer candidates with prior experience. Incumbents are more likely to be re-elected than their challengers. They have advantages such as easier access to campaign resources such as group endorsements, monetary funds, campaign workers, and issue information.