2
See this:
http://xoopsdocs.net/modules/docs/en/xu-003/online/ch02s05.html
Briefly, the weight is the value used by to sort the display order of blocks in a page. 0 sets the block first, 1 sets the block second, 2 sets the block third, and so on.
I usually weigh my blocks in increments of 10 in case I want to add a new block later, I don't have to renumber all the blocks.
HTH