Normally, each time you run an interactive plug-in, its dialog is displayed with all of the settings initialized to the ones you used the last time you ran it. This may be a problem if you made a mistake setting the values and you can't remember what they were originally. One way to recover is to exit GIMP and start again, but the Reset all Filters command is a slightly less drastic solution: it resets the values for all plug-ins to their defaults. Because it is a dramatic step, it asks you to confirm that you really want to do it. Be careful: you cannot undo this command.