I am a novice when it comes to championship shows and have heard mention of benchguards.  Can someone try and explain to me what these are please

They are like one side and a top of a cage they have U shaped pieces that drop onto the top on the seperators of the benches. You can if the bench floor has holes or slots lock the front in place but some do not have any holes so locking them is a bit difficult if not impossible. The design may have changed I have had mine for a long while.

Of course if yoru neighbour is using one too it makes it harder.

I brought one just before Crufts, they can be locked onto the seperation panels at the side as well as on the top :)
Mind you mine was faulty (sent it back - got replacement) but there was a point at crufts when I thought I would be sneaking out with the panel attached  as the screw locked hard and I couldn't shift it! :)