A lot. A pollster estimates the percentage of the vote a candidate might win on election day. Players on PredictIt estimate the chance that a candidate will win the election.  For example, a candidate in a two-person contest might be leading her opponent 60% to 40% in the polls.  Candidates who are leading by a margin this large almost always go on to win. Consequently, PredictIt may show a probability for the leading candidate of 95% or higher.