Join Date: Nov 2007
Location: SF Bay area
Thanked 6 Times in 5 Posts
Rep Power: 8
As mentioned it's an attachment point for your mainsheet, which typically is arranged in a block-and-tackle (not in the picture) to get a mechanical advantage. The pair of dark gray plastic cams at the top forms a cam cleat: if you pull a line through it towards the flat side, it will open, but if you apply tension in the other direction, the cams will close down and pinch the line.
Do you have a picture of the outboard end of your boom? That's the other attachment point for the mainsheet.