I don't know exactly what they're getting at, but it might be the way an "object" can have a "data block", which is mesh data, or a curve, or what have you (an object that doesn't have a data block is an "empty"). Multiple objects can share the same data block, which can be useful. I use this in an early stage of sculpting hands, sharing all the finger data, then I set them single-owner for individual finger shaping.
It's useful for reducing the amount of work you need to do in some specific situations, and it can be useful for initial stages when sculpting things that are very similar but can't be accomplished with a mirror modifier, like when you're sculpting a posed figure and want to do the legs, arms, and other non-mirrored symmetrical bits at the same time for any portion of the work.
It's very important to learn if you want to do some scripting.
Honestly, one of the best things you can do is learn a bit of the scripting. Blender has a lot of little interface annoyances you can overcome with some simple scripting, and a lot of things don't have full plugins available. Here are three little scripts I wrote up while working on my last small project. The top one makes all the modifiers of all selected objects non-visible (mostly so I can work on one side without a mirror modifier interacting with Orbit Around Selected). The middle one sets all selected objects to have the remesh size of the active object, and remeshes all the objects. I use this when working on things that have a lot of different objects, especially hands, and I want to remesh them all at once at the same voxel size. The bottom one just unlinks all the mesh datas, making them all single-user. This is how I take a bunch of linked parts and unlink them in bulk, because in Blender alone you have to do it one at a time, which gets really old.
Learning a bit of the scripting can save you a lot of work: https://docs.blender.org/manual/en/latest/advanced/scripting/introduction.html