Consider the ranges A1:A3 and B1:B3 having the same size and only number values, as below

=SUMPRODUCT(A1:A3,B1:B3)

This will loop through the ranges, taking the product of values in the same row and summing them, returning 32 in this example.

A1*B1 = 4
A2*B2 = 10
A3*B3 = 18

Consider the following ranges A1:A3 and B1:B3 as below

=SUMPRODUCT(--(A1:A3="c"),B1:B3)

This will first manipulate (A1:A3="c") into the following array

A1="c" = FALSE
A2="c" = FALSE
A3="c" = TRUE

Then apply the -- operator which converts TRUE and FALSE into 1 and 0, respectively. So the array becomes

--FALSE = 0
--FALSE = 0
--TRUE  = 1

Then the SUMPRODUCT formula completes as in the simple numeric case. Returning 6 in this example

0*4 = 0
0*5 = 0
1*6 = 6

Note: this is the equivalent of a SUMIF function