What exactly is a "wireless" zone and how does it relate to security systems?

yes. or door or window sensor or glass break sensor or smoke alarm. generally 1 device = 1 zone with wireless.

It means that the system is capable of using wireless transmitters. Sometimes it's necessary to add some modules to add wireless capability to a panel. Depends on the mfg and the model. When using wireless devices, each transmitter is a zone. If you are looking for details here, or in any tech group, it's helpful if you tell people what equipment you're working with.

is a "wireless" zone in regards to security systems? Is this like a motion detector or what exactly?.

Zones on wired systems are simple circuits which can contain one or more sensors. Supervised wireless systems assign a single sensor to a zone. Unsupervised wireless systems can have multiple sensors on a zone. Note, however, that almost all modern wireless alarms are of the supervised variety.