A ZenPack is a plug-in or extension to Zenoss. Zenoss has many (more than 400) ZenPacks available. Typically, ZenPacks add the ability to monitor new types of devices, but can also be used to add new capabilities to the software itself, add new reports, etc. The extensibility available via ZenPacks is a critical part of Zenoss, as datacenters have lots of different types of hardware and software (such as cloud services) that need to be monitored, and ZenPacks allow you to monitor everything!
ZenPacks are typically released as Open Source software, so you get to benefit from ZenPacks that others have written. Zenoss also releases many very sophisticated ZenPacks as Open Source software.