The ZFS level-2 Adaptive Replacement Cache (L2ARC) uses block storage devices to effectively expand the size of the ARC. For best practice, the L2ARC devices should be much faster than the main pool devices for random reads. Hence, it is common to see SSDs used for L2ARC when HDDs are used for the main pool.
Each entry in the L2ARC takes some space in the ARC, so there is an effective limit to the size of the L2ARC that is dependent on the average recordsize of the data and the size of the ARC. A simple spreadsheet can help determine the ARC space requirements for L2ARC devices and recordsize.