Shaving Brush Knot Length - Loft
The loft simply describes the length of the knot, from its visible base to the tip.
Shaving brushes with longer lofts will give you more flex in the bristles, to the point where they can become floppy. Whereas a shorter loft will provide more stability to the bristles.
Finding a length (LOFT) that suits you is simply a matter of experimentation.