what is this?

It means someone typed a word that the content filter doesn't like. Sometimes it is obscene, sometimes it is something like br*ast cancer. The filter can't tell the difference and jsut reacts to the actual word typed.

Mature content means if you use word like s*x,abo*tion in your post.
If you want to use these words in your post,or want to respond to this type of post....
Then you have to allow mature content in your preference page.
To do this go to your edit your preference page.
Click allow mature content box.
Save your preferences.