Maturity ratings are defined for regions and thus apply to parcels. They are and indicator of the type of content which may be found there. Events also have a maturity rating.
Maturity ratings are one of the following:
G - General: (Formerly “PG”) A region designated General is not allowed to advertise or make available content or activity that is sexually explicit, violent, or depicts nudity. Sexually-oriented objects such as “sex beds” or poseballs may not be located or sold in General regions.
M - Moderate: Second Life's Moderate designation accommodates most of the non-adult activities common in Second Life. Dance clubs, bars, stores and malls, galleries, music venues, beaches, parks, and other spaces for socializing, creating, and learning all support a Moderate designation so long as they do not host publicly promoted adult activities or content and do not use adult search tags. Groups, events and classifieds that relate to this broad range of activities and themes generally should also be designated as Moderate.
A - Adult: The Adult designation applies to Second Life regions that host, conduct, or display content that is sexually explicit, intensely violent, or depicts illicit drug use.
For more details, please consult this SL Wiki page.