Countering is a lance move, which blocks the attack of the monster and hit it. It may not be very usefull if you play in teams (because the monster must attack you), but it is really great if you play alone. I posted 2 videos of a guy who did the barroth, barioth & rathalos arena alone in under 13 minutes in this thread a while ago. If you watch the video, you can see countering in action. He counters nearly every attack.
To do a counter attack you hold the wiimote vertical and press the A-Button. Best to do it right before a monster attacks you. I started countering just recently and I am still not very good in it. However I used it in my last arena fight against Rathalos runing attack or however it is called. And I won and needed only 41 minutes this time. So I got finally the third barioth coin and now I am the proud owner of a scholars blouse plus.
And don't listen to Soriku. The reason he has seen nobody uses countering could be that he only plays with non skilled lancers or that he hasn't recognized the counter attacks. Here is a quote from an FAQ:
"Let's move on to countering now. This lance technique is looked as the sole reason as to why lance is the single best weapon in Monster Hunter Tri."
The whole FAQ can be found here: http://www.gamefaqs.com/wii/943655-monster-hunter-tri/faqs/60132
