Do not understand sumproduct formula

From the attached spreadsheet, I am trying to determine how many patients show up between:

8:30a and 8:30p -
8:30p and 8:30a -

8a and 8p -
8p and 8a -

7:30a and 7:30p -
7:30p and 7:30a -

I don't understand the formula =SUMPRODUCT(--(LEFT($L$2:$L$108,2)=U2)) for beginners....I have the hours, but how do I get it to calculate by the half hour. I've tried a few things to no avail. Would someone mind just telling me with this formula means in terms a person can understand :-)

THANK YOU!!!!