Countercurrent exchange is a mechanism occurring in fish gills and lots of other places Biology, such as the loop of Henle in kidney nephrons. It’s also used industry and engineering in such things as heat exchangers, in which there is a crossover of some property, usually heat or some chemical, between two flowing bodies flowing in opposite directions to each other.
Fluids flowing in the same direction (concurrent flow) only exchange materials until there is an equilibrium, both having a maximum of 50%. When the flow directions oppose each other (countercurrent flow) transfer does not stop at 50%. In fish gills the blood with 50% oxygen flows past water with more oxygen and the transfer carries on.
In concurrent flow there is a rapid exchange that stops when both fluids have the same amount. In countercurrent flow there is a slower initial exchange but this carries on for longer since the equilibrium betweeen the fluids is avoided for longer, until the blood is fully saturated with oxygen.