SPD = Serial Presence Detect
It is and EEPROM chip that stores information on the DIMM modules' size, speed, voltage, drive strength, and number of row and column addresses. When the BIOS reads these parameters during the POST routine, it automatically adjusts values in the BIOS Chipset section for maximum reliability and performance.
I choose to diasable it and run more agressive timings. Experiment and see what works for you.