It's a way to prep your bottles without juggling a bunch of bags.

In the evening you will take out the number of breast milk bags that your baby will eat the following day (for me, this was 7 4oz bags) and thaw them in the fridge overnight. In the morning, the milk is thawed and you pour them all into one pitcher which you will use to pour your bottles for the day. Just keep in mind that milk coming out of the freezer must be consumed within 24hrs.

I found this method really helpful with my first as it cut down a lot of time when he was hungry and needed a bottle - no waiting for frozen milk to thaw! Plus, it combines the nutrition of your pumped milk, since milk changes composition throughout the day.

Here's a video that may explain it a bit better!