What is dual space, in simple words?

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I noticed that the rank of a tensor is a kind of "two-dimensional" property - the covariant components come first, then the contravariant. If I understand correctly, these components refer to the normal vector space and its dual counterpart. What is it, and why does vector space have some kind of dual counterpart, in simple terms? (Also, why is the norm of a vector space and its dual related by the formula 1/a+1/b=1?)