How to Enter Formulas and Functions in the Excel Formula Bar
When you place your entry in the Excel Formula Bar, the entry is really going into the active cell. However, because the active cell can be anywhere, you may prefer entering formulas and functions directly in the Formula Bar. That way, you know that the entry will land where you need it.
Before you enter a formula in the Formula Box (on the right end of the Formula Bar), the Name Box on the left lets you know where the entry will end up. The cell receiving the entry may be not be in the visible area of the worksheet. Gosh, it could be a million rows down and thousands of columns to the right!
After you start entering the formula, the Name Box becomes a dropdown menu of functions. This menu is useful for nesting functions. As you enter a function in the Formula Box, you can click a function in the Name Box, and the function is inserted into the entry you started in the Formula Box.
This is a helpful way to assemble nested functions. Try it, and get used to it; it will add to your Excel smarts.
When your entry is finished, press the Enter key or click the little checkmark button to the left of the Formula Box.
The following figure makes this clear. A formula is being entered in the Formula Box, and the Name Box follows along with the function(s) being entered. Note, though, that the active cell is not in the viewable area of the worksheet. It must be below and/or to the right of the viewable area because the topleft portion of the worksheet is shown here.
In between the Name Box and the Formula Box are three small buttons. From left to right, they do the following:

Cancel the entry.

Complete the entry.

Display the Insert Function dialog box.
The Cancel and Complete Function buttons are enabled only when you enter a formula, a function, or just plain old values on the Formula Bar or directly in a cell.