In short: They are special air paths named the dado panels or decompression panels:
In normal conditions they are partially open to allow air entering the cabin from the ceiling (crown) to exit into the cargo holds.
In case of rapid decompression from a hole in the cargo holds, they fully open to accelerate pressure equalization between the two levels and prevent the cabin floor to collapse down under the effects of high pressure differential.
They are standard safety equipment since a NTSB recommendation after a DC-10 explosive decompression in 1972, due to a cargo door opening in flight. The cabin floor collapsed:
disrupting various control cables which were routed through the floor beams to the rear engine and to the empennage control systems
In the principle they are similar to the louver panels found in restroom doors (doors may be undercut instead). They play a secondary role as firewalls.
In the architecture lingo, the dado is the lower part of a wall below the dado rail. The grille portion of a passenger aircraft wall is called the dado module. It consists on a louvered air grille over a decompression panel (sometimes called the dado panel) which can be open or closed.
Boeing patent for a decompression panel, source
The cargo compartments on a pressurized aircraft are not always fully heated, but they are always pressurized, being part of a single pressure vessel (a cylinder sealed by two bulkheads).
Air from the packs (conditioning system) enters the cabin usually from the ceiling and leaves it by the dado modules, e.g. (A320):
Cabin air distribution, source Airbus
Longitudinal flow, schematic principle, source
There are recirculation ducts in the cargo section with fans drawing air from the cabin. Air is then reconditioned and sent back to the cabin. A fraction is dumped overboard though the outflow valves to regulate pressure in the aircraft.
In case of decompression
In case of rapid decompression in a cargo area, there is a risk of cabin floor collapsing due to the pressure difference above and below it. This happened in 1972, as @krmezljavKuza mentioned. The related NTSB investigation report included this recommendation:
Require the installation of relief vents between the cabin and aft cargo compartment to minimize the pressure loading on the cabin flooring in the event of sudden depressurization of the cargo compartment.
The decompression panels open to act as pressure relief vents and allow a larger quantity of air to flow into the cargo compartment. Pressure can equalize before the floor is damaged.
There are similar decompression panels in the triangle areas indicated in the picture above, between the different sections of the cargo level. There are also louvers in various panels, e.g. lavatory doors.