'Extreme' here refers to extreme violence. ' Explicit' is very visible, clearly depicted (violence or porn) In some sense, they are fairly interchangeable in this context, but have subtle differences in nuance.

Generally speaking, explicit content relates to sexual content that isn't violent, whereas extreme content relates to violent, disturbing, or sexual content of a violent nature.

This week’s controversial topic is ... ‘violent video games’. Are they responsible for a rise in the number of attacks by children in the schoolyard?’ Some social commentators say yes. Worse, it has been suggested that two recent killings by teenagers were prompted by the playing of video games with extreme content. But is it proven?