Water melted from the evaporator during defrost runs through a drain tube to a pan outside the cold space, usually in the machine compartment. Here it is evaporated away by heat: a loop of hot discharge pipe running through the pan, warm air from the condenser fan, or a small electric heater. Most plug-in cabinets work this way so they do not need plumbing.
The pan must keep up with the water arriving. In humid weather and with frequent door openings, more frost forms and more water drains out. If the pan overflows, water spreads under the cabinet, which leads to smells, corrosion and slip hazards. A blocked drain tube can also send water back into the cabinet instead.
Cleaning the pan is part of good hygiene. Food particles and dust in warm standing water grow slime and odours. Check it during routine condenser cleaning, wipe it out, and make sure the drain tube outlet is sitting in the pan and not dislodged during moving or cleaning.