SUM adds up a range of cells. =SUM(B2:B9) adds B2, B9 and every cell between them; the colon means "through". To add several ranges or single cells, separate them with commas: =SUM(B2:B9,D2:D9).
AutoSum writes the formula for you. Select the empty cell under a column of numbers and click AutoSum (the Σ on the Home tab), or press Alt+= on Windows. In Excel for Mac the shortcut is Cmd+Shift+T, which your browser keeps for reopening tabs, so click the button here. Excel guesses the range and shows it with a moving border. Check the guess, then press Enter.
SUM skips text and empty cells, so a stray label in the range won't break it. Still, start the range at the first number so the formula says exactly what it adds.