An SOCD cleaner prevents controllers from registering opposite inputs 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 in both directions when playing games like Street Fighter, but because most if not all hitboxes have an SOCD cleaner, only one direction registers. More specifically, pressing back and forward at the same time would result in neutral. As for up and down, the result would be up.
What is SOCD?
SOCD stands for simultaneous opposing cardinal directions. It is important because most major tournaments ban controllers that do not have this functionality. Fortunately, mainstream hitboxes and similar controllers, like mixboxes, have this feature, so you should not worry. If you would like to convert an arcade stick into a hitbox or mixbox, consider getting an SOCD cleaner kit.
If you would like to start using a hitbox, check out our relevant top hitboxes list.