When you're ready to total all your work hours at the end of the week, you would use the SUM function again, but this time you wouldn't have to type the formula.
You could get the total by selecting cell G6 and then clicking AutoSum on the Standard toolbar to enter the SUM function. Excel will enter the formula for you:
If you press ENTER, the SUM function will add all the values in the range of cells from B6 through F6 (the colon indicates a range). The total is 40.25.
When you click AutoSum, Excel looks for a contiguous range of cells with numbers in them, either in the column above or in the row to the left. The range it selects is highlighted with a color marquee, which gives you a chance to choose a different range in case Excel guessed wrong.
Note You could type the formula directly into the cell, but you might type it wrong, and the AutoSum button is faster.