Normally when you email something, you can't know if someone opened it. Because you basically tell the email company, "tell them I said X".

So to get around this, you put in a specific address to a website that you control.

When someone opens the email, it'll be told "automatically load the picture from this website", where the picture is a single dot. Their email program "fetches" the dot.

When you go to check to see if they read the email, you can see when they opened the email because that's when the email program asks your website to show the picture (and logs it).

What this guy is saying is that instead of asking your website to keep loading that dot each time, it'll do it the first time and then just save the picture.