Open Shift Bids are shifts that have been posted-up for individual bid by your Supervisor. Unlike the Trade Board, these Shifts are not first-come-first-served. You and your Co-Workers bid on an individual Shift, and then your Supervisor determines how the Shift Bid is awarded, either by seniority or other method.
You can utilize the tab navigation at the top of the RosterApps page to get to the Open Shift Bid Page.
You can see that there are two shifts that are up for bidding displayed in the calendar.