Content levels indicate what you're comfortable with happening in a roleplay. Specific OOC communication between players is always recommended, but these are to function as a universal baseline. Specific personal player limits always override content ratings
PG-13: Minimal violence, minimal sexual content. Both can be present, but not explicit.
R: Explicit violence is limited to fights between characters, but otherwise unrestricted. Sexual content is limited to consensual encounters.
NC-17: Violence does not have to be combat; torture / sadism is allowed, and can be sexual in nature. Sexual intercourse must be consensual.
X: No restrictions on violence. Non-consensual or coerced sex is allowed.
The different security levels are designed to have different tones and general feels.
MIN: Orange is the New Black; More focused on interpersonal drama and shenanigans between inmates. Enforced summer camp with handcuffs.
MED: Prison Break; Dealing with the dark and gritty hardships and realities of prison life.
MAX: Oz; Dealing with the psychological torture of high security, whether fighting it or living with it.
SMAX: Sexy Guantanamo; Being broken down, tortured, and turned out for the amusement and profit of the prison. Or at least they'll try.