“Cumulative” sentences mean different sentences that are added together to get a total sentence length. This will usually be because the offences were of a different kind and not connected. The judge will state whether or not sentences are concurrent when sentencing takes place.
For example: Someone is sentenced on one charge of assault and one charge of burglary. Each charge was unrelated and took place on different days. They are sentenced to 3 months imprisonment on the assault and 4 months imprisonment on the burglary, to be served cumulatively. The total prison sentence will be 7 months.