It means that it's the simplified version.
The image is of having just enough of something to fit into a nut shell. Figuratively, it means to give just the basics, only what is absolutely needed.
Nut.
A nut is a hard-shelled dry fruit or seed with a separable rind or shell and interior kernel according to webster's
The cashew nut
Walnut is an edible nut with a smooth thin shell.
The nut with the hardest shell is the macadamia nut. It is difficult to crack because the edible part is stuck to the inside of the shell.
Shell.
No.
A coconut.
nut
testicle
a doughnut because they are baked and dough nut that says it all that not shell but dough