Multi-port fuel injection is any type of fuel injection system that has multiple injectors, located in the intake ports, which makes the distribution of the fuel/air mixture more even among each combustion chamber. This reduces the chances that one cylinder might get too little fuel, while another might get too much at the same time.
In some new high fuel efficency cars, this system allows the ECU to stop injecting fuel into certain cylinders. This is called cylinder deactivation, and is sometimes used on direct injection engines, but the goal of reducing how much fuel is consumed is the same for both types.