This board is an inexpensive 3.3V to 5V I2C Translator and Hub Board.
It allows you to mix 3.3V and 5V I2C Devices in the same design. It also provides additional 3.3V and 5V Grove I2C connectors. You can also use it as a 6 port I2C hub for either 3.3V or 5V.
The 3.3V to 5V I2C Hub Board is easy to use. Typically, one I2C bus will be connected to 5V and the other to 3.3V. The main connector to the computer will typically have 3.3V in the case of a Raspberry Pi) or 5V (in the case of most Arduinos or the Pi2Grover Interface Board). The other side of the bus (with all the I2C devices on it), typically is unpowered and you will either need to hook up 3.3V or 5V depending on what bus you are talking to.