A blow-through configuration refers to how the coils and the supply fan are configured in an air handling unit. In a blow-through configuration, the coils are installed downstream of (after) the supply fan. This means that they are installed on the “discharge” side of the fan.
On the discharge side of a fan, air is being pushed. The supply fan will “blow” air “through” the coils in this configuration.
If the coils and fan were to switch places, it would be a draw-through system. You can learn more about that by clicking here.
The image below shows a standard blow-through AHU configuration. Notice the fan would push (or blow) air through the heating and chilled water coils.