I've waited a long time for Granger to get into a situation where ze wants to use that spell!

In the books, they say that the Killing Curse is unblockable. Presumably, what that means is that the Killing Curse is too powerful to be blocked by magical shields. Literally speaking, it gets blocked by various things, including a phoenix, an animated stone statue, and the power of love. Granger doesn't like relying on lucky circumstances, so ze decided to develop a spell that could block the Killing Curse reliably. In Voldemort's Children, love can't block curses, so ze had to stick to using a physical object.

So, Granger invented “Aegidivitrea”, the Glass Shield Charm (ze's very practical about naming things). It conjures a thick shield of glass around the caster and anyone else the caster wishes to protect, and continuously keeps the glass hovering in the air. Since it's a solid object, almost all curses will bounce off of it. The easiest way to attack the shield is with a spell like the Blasting Curse ("Confringo"), but those spells can be blocked normally. When coupled with the regular Shield Charm, as the three Aurors are doing here, it's very hard to attack.

The main catch is that conjuration is permanent in this universe. It's easy to make the glass disappear again, but if you become incapacitated somehow and can't do that, there's a ton of unsupported glass ready to fall down on top of you. So the Aurors only use the spell when they are travelling in groups of two or more, so that they can cover for each other. And fighting in groups is the ideal way for Aurors to operate in any case.

(You might think there's another weakness in that the shield blocks spells you cast from the inside just as much as it blocks spells your opponents cast from the outside, but it's easy to create little holes in the shield to cast spells from, while your opponents will have a very hard time hitting those holes accurately.)