The Hatter is explaining that it's impossible to take less than nothing, which humorously highlights the absurdity of certain logical ideas. He points out that while taking more than nothing is straightforward, reducing something to less than nothing isn't possible, reflecting the whimsical logic of Wonderland.
"You mean you ca'n't take less," said the Hatter: "it's very easy to take more than nothing." This quote showcases the playful and nonsensical nature of Lewis Carroll's writing, where traditional reasoning is turned upside down to create amusing and thought-provoking moments.