The ratings system is a useful thing to help you determine whether the game is suitable for you or not. It very simply rates the violence, the gore, or the language (or all 3) on a scale of 1-10, as follows:
Clean and suitable for all.
Contains very mild violence.
Contains very mild violence and gore.
Contains less mild violence and gore, and/or light cursing, eg d*mn, h*ll, etc.
Contains mild-ish violence and gore, and/or more regular light cursing.
Contains moderate violence and gore, and/or lesser-moderate cursing, eg sh*t.
Contains harsher violence and gore, and/or greater-moderate cursing, eg f*ck.
Contains severe violence and gore, and/or repetetive use of sh*t, f*ck, etc.
Contains very severe violence and gore, and/or severely offensive curse-words.
Contains horrific graphic violence and gore, and/or severely offensive curse-words.
As well as violence, gore and language, the ratings rate the concept/theme of the game, 1 being a very innocent and simple concept, 10 involving drugs and other adult themes.