Cruft is useless stuff, not exactly crap, but more like the lint in your pants pocket that used to be part of the pocket material, or clothes-dryer fluff. Or the random scraps of material that are left over after you whittle down a piece of wood.
Think more like "useless" than like "crap".
I've seen it spelled "kruft" also.
Thanks. Now I will probably encounter this word several times over the next several days.