conkers

[stbjohn2]

I just followed a link on muggle.net to a report on filming of GOF. 
(they are somewhere in surrey, filming at a park with a lake, so 
maybe it's the 2nd task). The blogger said he/she(?) saw Emma Watson 
wearing a heavy-duty protective mask playing conkers. So what the 
heck is Conkers?  (sorry if it's been discussed before, I did a 
search until my patience ran out and couldn't find anything)
Sandy