This is a tricky use of N() that allows you to use it as a way to leave in-cell comments. It only works for formulas that return numeric results.
The N function takes a value and returns a number. When given a text value, the N function returns zero.
In this case, the primary formula the SUM function, and the N function simply evaluates the comment text and returns zero:
=SUM(F5:F8)+N("Q4 numbers are estimates") =SUM(F5:F8) + 0 =410,750
The comment is visible in the formula bar when the cell is selected.