That's because you just passed the requirements to unlock it
It's 150 rep to unlock it, AFAIK.
Regardless, "mark as held" is a feature that when you check it, it will hide the post from the general membership and queue it for moderation. This means the members CANNOT see the post that is held, but the moderation team can, that way we can take the appropriate course of action.
The "mark as held" tool should only be used for extreme circumstances such as, and for example: Porn, Gore, Spam bots / spammers / things of that nature. Do not use it for any other "lesser" reason. For those sort of things, stick to the traditional report button method without checking the "mark as held" box.
