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If you are referring to Kagamine Rin & Len, the answer is already explained on their wiki page.

Basically, the two were supposed to be twins but the idea was dropped by Crypton before their release. They were then announced as "mirror images" so that fans could interpret their relationship any way they wanted for a song.

A mirror image is in simple terms someone who looks just like you, for example, how Len looks like a male version of Rin.