Logical Volume Management (AKA LVM) is a powerful, robust mechanism for managing storage space.
In TurnKey 11, instead of installing the root filesystem directly to a fixed size partition, we setup LVM by default, and install the root filesystem to a Logical Volume, which may later be expanded, even across multiple physical devices.
Unfortunately, as with anything powerful, to get the most out of LVM you first have to negotiate a learning curve. From the feedback we've been getting it seems that confusion regarding LVM is common with new users, so here's a quick "crash course"...