The grid can be used to find items in the Product Backlog and then assign them to a Sprint.
You can filter to show only those items in the Product Backlog.
Simply click on the sprint cell to set the Sprint for the item.
If you have selected multiple (or all visible) rows with the checkbox on the left of the grid, all selected rows will be updated with the value you set
You can click to assign a sprint value for any grid item.