A head-to-head table simulates head-to-head match-ups based on respondents' rankings. A candidate ranked higher than another candidate on a voter's ballot is assumed to win that voter's head-to-head match-up. All ranked candidates on a ballot are assumed to be preferred to all non-ranked candidates.
This chart is best read across rows. For example, the Joe Biden row can be read as: "Joe Biden wins 59% of head-to-head match-ups against Michael Bloomberg, 66% of head-to-head match-ups against Pete Buttigieg..." and so on. As shown, Bernie Sanders wins a majority of a head-to-head match-ups against each other candidate.
See a larger table here.