Explanation
The key is this snippet:
--(B5=things)
which simply compares the value in B5 to every value in the named range "things". Because we are comparing B5 to an array (i.e. the named range "things", E5:E11) the result will be an array of TRUE / FALSE values like this:
{TRUE;FALSE;FALSE;FALSE;FALSE}
If we have even one TRUE in the array, we know that B5 equals at least one thing in the list, so, to force the TRUE / FALSE values to 1s and 0s, we use a double negative (--, also called a double unary). After this coercion, we have this:
{1;0;0;0;0}
Now we process the result with SUMPRODUCT, which will add up the elements in the array. If we get any non-zero result, we have at least one match, so we use >1 to force a final result of either TRUE or FALSE.
With a hard-coded list
There's no requirement that you use a range for your list of things. If you're only looking for a small number of things, you can use a list in array format, which is called an array constant. For example, if you're just looking for the colors red, blue, and green, you can use {"red","blue","green"} like this:
--(B5={"red","blue","green"})
Dealing with extra spaces
If the cells you are testing contain any extra space, they won't match properly. To strip all extra space, you can modify the formula to use the TRIM function like so:
=SUMPRODUCT(--(TRIM(A1)=things))>0