Heat exchangers with multi pass flows means that the fluids pass through the heat exchangers several times before exiting. This type of flow creates a thermally longer heat exchanger, while still maintaining the same compact product footprint.
The most frequently used version of this is the 2-pass execution, where the flow travels 2 times the plate length before exiting on the other side. Due to the flow arrangement of multi pass units, the inlet and outlet connections must be on opposing sides. This 2-Pass model can be used for various applications, but most predominately in applications such as Liquid CDUs for Data Center cooling and C02 Gas Coolers.
Here is a short animation showing the flows within a 2-Pass BPHE: