I read about this sentence first time while browsing Wikipedia about Steal Like an Artist, the idea is based on a book with same name on coming up with creative ideas, by Austin Kleon.
What it means?
The closest I can think of is in the design of products like smart phones, instead of having dozens of buttons you just have few home and navigation buttons.
In an age of information overload and abundance, focus is important. Choose what you want to leave out of your key work. “Nothing is more paralyzing than the idea of limitless possibilities. The best way to get over creative block is to simply place some constraints on yourself”, says Austin Kleon author of Steal Like an Artist.
A good example of design by constraints is Dr Seuss wrote his bestselling book “Green Egg and Ham” with only 50 different words (source). Saul Steinberg says a work of art represents a struggle against limitations. Creativity isn’t just the things we choose to put in, it’s the things we choose to leave out,” advises Kleon.
If the subtractive principle of art is still not clear, let me leave you with the image of a Zen garden where a single stone and a bit of raked sand convey oceans of meaning.
Creativity is subtraction
it helps eliminate the un-neccessaties of life
sometimes, at least
less is more
Simply Simple 
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