Help on IF Formula evaluating multiple criteria

Ok, I'm having brainfreeze right now and cannot remember how I got this to work. Below is a formula from another workbook I'm currently using that works as intended:

{=SUM(IF($D$4:$D$5000=L6,IF($E$4:$E$5000=1,1,0)))}

My only question is HOW I got those outer brackets around the formula to get it to work. I can't recall the term used for this type of formula and my attempts at using the Excel Help menu to find the answer weren't successful. Thanks in advance!