Hidden emoticons are the icons that cannot be found in regular emoticons menu of your favorite instant messenger, even that they are regular emoticons. Because that invisibility, they are also called hidden emoticons. To see them, as normal visible emoticons, you should type their own keyboard letter (code). After you type the secret code, hidden emoticon will be shown in your messenger and your friend will also see that emoticon or smiley right after you send that code.
Purpose of hidding emoticons from menu?
Hidden emoticons always be there with any messenger but because their owners think that they are less used than other emoticons, they hid them from IM users. They provide users code that can add this emoticons so they were no need to adding them and use space in IM smileys menu.