An accumulative judgment is a judgment that a criminal receives after it is discovered that they have committed a crime other than the one(s) they have already been convicted and sentenced for.
The sentences for accumulative judgments do not begin until after the sentence for the first crime has already been served. So, for example, if a person is convicted of arson and it is later discovered that they committed grand theft as well, the accumulative judgment for grand theft would take place after the first sentence is served. So, in this case, the person could serve four years in prison for arson and then another two years for grand theft.