You could use the SUM() function to keep a running total. If you had the row total in Column C, you could put the running total in Column D:
=SUM(C$2:C2)
Then use autofill for all the rows below.
":" means you are referencing a range of cells. A2:D6 means the rectangle of all cells from A2 to D6. "$" means "preserve" which is useful for autofill. So in the example above, the default behavior is to update C2 to D2, then E2, etc. as you autofill the formula down, but by specifying "C$2" for the first cell reference, you tell Numbers that you always want Row 2 no matter where the formula is.
You can check out all of the formulas and functions from Apple Support: Formulas and Functions Help - Apple Support
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