Wikipedia - A brokered convention is a situation in United States politics in which there are not enough delegates ‘won’ during the presidential primary and caucus elections for a single candidate to have a pre-existing majority…
…thereafter, the nomination is decided through a process of alternating political horse-trading, and additional re-votes…
…all regular delegates (who, previously, were pledged to the candidate who had won their respective state’s primary or caucus election) are “released,” and are able to switch their allegiance to a different candidate before the next round of balloting. It is hoped that this ‘freedom’ will result in a re-vote resulting in a clear majority of delegates for one candidate.