What is meant by advanced dementia, very aggressive?

Advanced dementia can take many forms: delusions,inability to dress,bathe etc without direction and aid,inability to carry out simple instructions without prompting, such as "eat your dinner." without you first putting the fork in their hand.
Not all sufferers of dementia become physically, or verbally aggressive.However many will in extreme fear or panic have tantrums: screaming, kicking their feet etc.
Change in a room, a residence, over stimulation even the slightest touch to their body can trigger this behavior .It's all about fear and the brain no longer able to adequately process quickly and reliably whats happening to or around them.

Not sure what the context is. Have you talked to the doctor about what the recommendation is for level of care, going forward? Might hospice be appropriate at this point? I know how this feels; medical professionals say things to us and we reslize 20 minutes later that we (or at least I ) have no idea what they reaally mean.

It means they are attacking people. The people may be real or they may be hallucinations. If someone does this, call 911 for assistance.