Archive/Restore slots refer to the number of simultaneous archive/restore jobs that can be carried out by eMAM.
For example if you have LTO library with 10 tape drives, technically you can run 10 archive jobs and 10 restore jobs in parallel.
Some times, you can dedicate the tape drives for archive/restore jobs.
Based on that, archive/restore slots can be configured in eMAM. Most cases, this can be configured at the Middleware level.