The main section of the screen displays the individual cells in the form of a grid, with each cell being at the intersection of a particular column and row.
At the top of the columns and at the left-hand end of the rows are a series of gray boxes containing letters and numbers. These are the column and row headers. The columns start at A and go on to the right and the rows start at 1 and go on down.
These column and row headers form the cell references that appear in the Sheet Area box on the Formula Bar. These headers can also be turned off by selecting View > Column & Row Headers.