I'll give a quick review of this particular formula but to really
understand the in's and out's of this powerful function see
http://www.xldynamic.com/source/xld.SUMPRODUCT.html.

First break it in two parts:
SUMPRODUCT(($R$11:$AC$11<=Q2)*$R$14:$AC$14)
.....($R$11:$AC$11<=Q2)...creates an array of TRUE and FALSE values for
each cell in the range of R11:AC11 as that cell is compared to Q2.
....$R$14:$AC$14...creates an array of values from row 14
when multipling these two arrays the TRUE/FALSE change to 1 and 0.
SUMPRODUCT multiplies the corresponding values from the arrays then
adds the result together. For purpose of example, replace the dates in
row 11 with 1 for the Jan, Feb and Mar columns and zero for the rest
meaning that Q2 = March 1. The sumproduct formula would be (1*R14 +
1*S14 + 1*T14 + 0*U14...) which in effect is the sum of the all values
in Row 14 that meet the criteria.

SUMPRODUCT(--($R$11:$AC$11<=Q2*))
....This formula is basically a count of months that meet the
criteria...
Think of the -- as an operand that converts the TRUE and FALSE values
to 1 and 0. This happened automatically in the first half because of
the multiplication function. A better explanation is found in the link
mentioned at the start.

- John