If it is the block in the bottom of the boat in this picture, it is part of your mainsheet system. The lower block is call a fiddle block with a cam cleat. It's counterpart on the boom will be a single block with a becket. In sailing, pulley's are called
blocks. A becket is a small ferrule attached to the block. In this boat's mainsheet system, the line is tied to the becket on the boom end block, runs down to the smaller sheave on the fiddle block, back up through the sheave of the boom block then down through the large sheave that feeds the cleat. In a set up like this it would be handy to have a small stand up spring to hold the lower block upright. The spring is inexpensive and you can buy at the same place you purchase the replacement block.
Note.... Don't purchase too large of block... match it up to your mainsheet's diameter. The two Selden blocks pictured below run $45 and $24 respectively from Maui Pro Sailing.