Eager (and less-talented) designers often get confused about this instruction, turning it into: "It doesn't have instructions, therefore it's simple.
Consider a hotel shower. It has 11 things that might be dials, and five that actually are. The alert person, standing under cold water, at 5 in the morning, in a dark hotel room, will probably (???) realize that the bottom dial, all the way near the floor, is actually the one that controls the temperature.
The lack of instructions doesn't make something simple.
I used to write the manuals for the educational software we shipped in the mid 1980s. The goal was clear: write exactly enough that no one would call us on the phone.
Today, of course, instructions are really cheap to provide. On a shower, all you need is a simple label. But just about anything else you produce ought to come with digital instructions, written or on video.
Don't make us read your mind.
[Yes, it's true, almost no one reads the instructions… people are so self-absorbed and hurried that they plunge first. One more reason to build something simple. But at least you can post instructions so that after they fail the first time, they have a shot at getting it right the second time.]
PS if you truly care, list your phone number/email address on the instructions. Not an unattended mailbox. You.*
(*the single best way to improve just about any communication…)
Your designs (and your instructions) will get better faster.
[I limit myself to just one post per year about how bad hotel showers are, fwiw. Mostly, they're a symptom of a significant lack of care in the face of the rush to make more stuff faster.]