Ok it works like this.
You need to put your website URL on your bio page...
Google will crawl this forum website and see that you have a link here. (bio page only)
Now the google bot knows you have another link pointing back to your website.
Thus giving your website more leverage, importance, in the google rankings.
Of course you need lots of good links pointing back. Not just this one. ( No offense Lisa )
Remember do NOT put your URL on the main forum thread unless it really relates to the subject matter.