Venting adds copper which isn't connected to any nets on the board and does not affect the functionality. The are various purposes for Venting (i.e Thieving) such as improving etching throughout the entire panel, to improve manufacturing efficiency, and more. Typically small copper circles, squares or even a solid copper plane will be added to larger blank spaces to even out the copper distribution across a copper layer.
Note: FAB 3000 includes several sample Panel Setup files (*.pan) which make use of various venting patterns. We recommend trying those samples and evaluating the venting pattern results.