In gameplay, there are four types of Cargo: Oxygen Tanks, Carbon Dioxide Scrubbers, Water Tanks and Batteries. Each is depicted as a small, brightly-colored box, labeled as to its particular contents. Cargo is any essential material or substance needed to sustain the crew during their stay in space.
Once a module containing Cargo reaches space, the cargo can be swapped, moved or disposed by selecting the Cargo/Consumables tab in the VIEW section then performing the action you wish to complete.
Several different spacecraft can be loaded with cargo during the rocket assembly stage, such as: Automated Transfer Vehicles, Resupply Modules, LOK Crew Modules, LOK Docking Modules, Crew Modules, Crew Module Dockables and several others.
As a general rule of thumb, if the module can carry crew members, it will probably carry cargo as well. Stations will need to be re-supplied with new/fresh cargo periodically, as the previously supplied cargo is depleted (unless you want the station to power off).
A very important fact to remember about Batteries in cargo slots: if you remove all the batteries from almost any vessel's Cargo/Consumable slots, the vessel no longer has power and is inoperable. This can be changed by docking another vessel and connect the power grid, or swap charged batteries into the inoperable vessel. Vessels that are pre-mounted with batteries usually requires power as well.