Racketeer Influenced and Corrupt Organizations (RICO) laws. These were originally made to go after organized crime bosses.
Crime bosses will have layers of minions directly committing crimes. This lets the boss say it wasn’t them. RICO provides laws to go after an entire criminal organization, including the bosses who direct the minions to commit crimes.
They are using the law in the Trump case because many of the direct crimes were done by minions. If they can prove that there was an organized series of crimes that were under the direction of boss Trump, they can hold him accountable for the crimes of the organization.