Active space refers to the spaces that can be used for active/powered equipment. Active equipment is typically larger and requires at least 19 inches of back space. Passive space refers to spaces that can be used for passive/un-powered equipment. A example of passive equipment would be a patch panel that does not require much backspacing. Active space can be used as active or passive because if need be you can put a shallow passive piece of equipment into the space with out issue.
In the below picture you can see one of two bays on a Z1000-AC4.