Bidirectional Logic Level converter (3.3V to 5V, 5V to 3.3V)
Lots of electronics use 3.3V level logic, whereas Arduino will often work with 5V. So it can be useful to convert the voltage of signals, so as not to damage parts of your circuit, if any parts require 3.3V.
We have two kinds of logic level converters - pick the colour, green or blue. They both do the same thing, but the green one has more converter channels.
The way it works is, you connect your 5V and GND to one side, and your 3.3V and GND to the other side, (far left for green, and the middle 4 for blue) and then communication on the remaining pins will be stepped up or down, from L1 to H1, or H2 to L2, etc.