It's an absolute reference: it means don't treat this as a relative reference if you copy/paste. If you copy that into the cell below to it, you'll get
=B10/$G$3
i.e. Excel has automatically updated the row in the formula for the new location of B9->10, but left G3 alone - i.e. it's a constant you've set once that you want to apply to all rows or cells in a table.