How do I calculate a running total that grows as I go down a column?

The key idea is that one end of the range stays fixed and the other end moves. $B$2 (with both the column and row anchored by $) always points at the very first row of data, no matter where the formula is copied. The second reference, B2 without any $, is relative, so when the formula is copied down to row 3 it becomes B3, then B4 on the next row, and so on, always pointing at the current row.

The result is a range that always starts at row 2 and always ends at the current row, so each row's total includes every value from the top of the column down to itself: row 5's formula sums B2 through B5, row 10's sums B2 through B10, and so on.