Suspending a zone allows you to temporarily stop serving data for a a zone without deleting the zone.
This can be useful for example if you are hosting the domain name for someone else, and they forgot to pay the bill...
You can suspend / resume zones in the DNS Records window by right-clicking a zone in the left list and selecting Suspend / Resume from the pop-up menu.
A suspended zone is indicated by a "paused" icon and a red zone name.
How Simple DNS Plus responds (or not) to DNS requests for names in a suspended zone depends on the settings in the Options dialog / DNS / Local Zones / Suspended Zones section.
When you suspend or resume a zone on a "super master" server, the zone's suspended status is automatically transferred to all Simple DNS Plus "super slave" servers. For more on "Super Master/Slave" see Options dialog / DNS / Local Zones / Super Master/Slave section.
If the secondary server is not Simple DNS Plus or if you are not using the Super Master/Slave feature, you need to remember to suspend/resume the zone on both primary and secondary DNS servers.