On the left of the Formula bar is a small text box, called the Name box, with a letter and number combination in it, such as D7. This is the column letter and row number, called the cell reference, of the current cell.
To the right of the Name box are the the Function Wizard, Sum, and Function buttons.
Clicking the Function Wizard button opens a dialog from which you can search through a list of available functions. This can be very useful, because it also shows how the functions are formatted.
The Sum button inserts a formula into the current cell that totals the numbers in the cells above, or to the left if there are no numbers above, the current cell.
The Function button inserts an equals sign into the selected cell and the Input Line, thereby setting the cell ready to accept a formula.
When you enter new data into a cell, the Sum and Equals buttons change to Cancel and Accept buttons
The contents of the current cell (data, formula, or function) are displayed in the Input Line, the remainder of the Formula bar. You can edit the cell contents of the current cell here, or you can do that in the current cell. To edit inside the Input Line area, click the appropriate part of the Input Line area, then type your changes. To edit within the current cell, just double-click the cell.