Pre-bagging describes how to tie the baby with a rounded back in the sling before putting it on your back. Babies and Newborns are best pre-bagged lying on the floor, while infants from sitting age also can be pre-bagged while standing while the child is sitting, for example, on the bent knee.
From walking age you can skip the pre-bagging. Then the child is directly bagged on the back. However, this requires a little practice.