This lets you write and read byte values from an sd card. It’s very simple to use. The schematic uses some resistors in a voltage divider to drop down the arduino 5v logic to the sd cards 3v3 logic. So all outgoing signals from the arduino are dropped to 3v3. Incoming signals to the arduino are fine because 3v3 is above logic 1 for arduino. The 3v3 voltage supply from the decimelia was used to supply power to the sd card.
Interfacing SD Card With Arduino: [Link]