A syndicate is an organization of people who are united behind a particular cause that is usually related to a business venture.
In the context of the law, organized crime is often committed by "crime syndicates." These crime syndicates are often different groups who work together to perform crimes in which they all have an interest.
The crime that is performed by syndicates is often of a larger and more pervasive scale than that performed by non-syndicates. The reason is because syndicates often have more resources, more organization and more manpower. So, for these reasons, crime syndicates can be of particular interest to the FBI or other law enforcement agencies.
Sometimes, if a member of a crime syndicate is caught, they will be offered a lesser sentence in exchange for testifying against other members of the syndicate.