All in Two Nappies (also known as AI2 cloth nappies) are a two piece modern cloth nappy system. There's an outer shell, and an absorbent booster that attaches to the shell.
They're different to pocket nappies, where the booster goes inside the nappy cover. With AI2 nappies, the booster usually pops on with press studs. The advantage of these is that you can easily add extra absorbency by switching out the nappy booster. And if baby just has a wet nappy, you can simply swap the booster rather than the whole nappy. Anything that cuts down on washing is a good thing, right?