What is victim-blaming? You can read much more in the document of course, but classic examples are following a case study of abuse by asking about the victim "what did s/he do wrong?" rather than focussing on the abuser, or responding to a real-life online incident with "well you shouldn't have been on that app anyway!".
Why does it matter? Of course it is wrong to make victims feel like it is their fault for being abused and this can perpetuate the trauma and prevent recovery. Also, where children and young people have heard victim-blaming language used as standard, they may be less inclined to ask for help and staff might be less likely to view an incident as a safeguarding matter.