If your system is communicating with multiple devices over I2C, and they share the same address (e.g. several identical OLED screens), you will find that you will have difficulty individually controlling each of the devices (because they all have the same address). This is where you will need one of these.
A multiplexer allows you to put one set of data lines and connect them to a bunch of other ones. Think of it like a train line selector.
This module uses the TCA9548A, an 8 Channel Multiplexer for connecting up to 8x I2C devices per multiplexer, and up to 8 multiplexers maximum per I2C line. That means one microcontroller can have up to 64 devices when using 8 of these! And because these are bidirectional, you can talk and receive communications from each of these devices.