In the help example, A2 to A4 contain 5, 15 and 30 respectively.
The example then says:
"
=PRODUCT(A2:A4) Multiplies the numbers above (2250)
=PRODUCT(A2:A4, 2) Multiplies the numbers above and 2 (4500)
"
This is merely illustrating that the arguments of the PRODUCT function can
be either cell references or numbers, or both.
From the earlier part of the help for that function:
"Multiplies all the numbers given as arguments and returns the product."
...
"Arguments that are numbers, logical values, or text representations of
numbers are counted; arguments that are error values or text that cannot be
translated into numbers cause errors.
If an argument is an array or reference, only numbers in the array or
reference are counted. Empty cells, logical values, text, or error values in
the array or reference are ignored. "