In the past, it has happened that the Heatsink Guide discussion board was flooded with messages advertising various stuff, to the point of making the board unusable.
It appears that these messages were posted in large number by automated SPAM robots.
Reading the number and typing it in is just a simple proof that you are actually a human and not an automated posting robot :-).
I'm sorry about the inconvenience, but it is certainly a smaller inconvenience than having the discussion board filled with SPAM on a daily basis. Also, I do not like forums where you have to register to write messages. This "number check" appears to me as a good compromise.