An SOCD cleaner prevents controllers from registering opposite directions simultaneously. For example, with a hitbox, which is a fighting game controller that has all buttons, you can press back and forward at the same time, which may enable you to block both directions if there is no SOCD cleaner.
Because most if not all hitboxes have an SOCD cleaner, only one direction registers (or the result could even be neutral, meaning no direction).
What is SOCD?
SOCD stands for simultaneous opposing cardinal directions. It is important because most major tournaments ban controllers that do not have an SOCD cleaning functionality.
Fortunately, mainstream hitboxes and similar controllers have this feature, so you should not worry. If you would like to convert an arcade stick into a hitbox, consider getting an SOCD cleaning kit. Furthermore, if you would like to start using a hitbox, check out our relevant top hitboxes list.