Aye, "mature" normally means that the group will often use swears and roleplay things that aren't suitable for all age groups (this is only allowed in private chats such as group, party, and whisper as Buffy said above). Different groups have different guidelines, so if you're thinking of joining a group with a "mature" label it's best to ask the group leader privately about what is and isn't allowed in that specific group
I'll be locking this up now as well since you seem content with the information given