Nix expressions are evaluated to produce derivations (among other values). These derivations when realized usually produce the compiled binary packages. Sometimes, realizing a derivation can produce a Nix expression representing another derivation. This generated Nix expression too needs to be evaluated to its derivation before it can be realized. This secondary evaluation is achieved by importing from the derivation being evaluated, and is called “import from derivation” or IFD.
For detailed explanation, see this blog post.