Okay, there are a few different ratings your story can get. If it is above a G, you are required to rate it either in the topic title, the topic description, or the first part of your post.
Here is an explanation of the various ratings:
G= Safe for all ages, no swearing, violence, or sex.
PG= Some non-graphic, mild-ish violence. Dark.
PG-13= Any two combinations of violence, swearing, or sex.
R= Violence, swearing, and sex.
Anything above R is not allowed. (that means no graphic sex scenes or graphic violence)
If you do not add a rating, the story will be assumed to be rated G.