I think that's your contribution to the total amount of time spent on those boards.
Very certain this is it.
If you take your current amount of posts, divide that by the total posts in the board, multiple by one hundred, it'll be similar.
Example; I have 309 posts in coding help.
My activity is 0.4987% in coding help.
61961 posts in coding help.
309/61961*100=0.498700796.