So, I was playing with Nix REPL:
nix-repl> x = ./.
nix-repl> y = ./.
nix-repl> x + y
/home/pierre/nixos-config/home/pierre/nixos-config
nix-repl> "" + x + y
copying '/home/pierre/nixos-config' to the store"/nix/store/4lxgb8kg3h25aykf7z2cf38g67nnq5zs-nixos-config/nix/store/4lxgb8kg3h25aykf7z2cf38g67nnq5zs-nixos-config"
So, something was added to the store. How can I undo this copy? I don’t know what I’m doing. 
Nix just copied the current directory into the store, you did not break anything. If you want to remove the path from the store, you can run the garbage collector.
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Why did it so? I don’t understand what is happening.
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That’s how string interpolation works for path variables. Hopefully this nix manual section will help. Specifically this:
A path in an interpolated expression is first copied into the Nix store, and the resulting string is the store path of the newly created store object.
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