Complementary for $SAT$
Usually, when we talk about the complement of a set we have some reference set to compare to.
In the setting of languages over some alphabet $\Sigma$, this means that the complement of some language $L$ would be $\overline L = \Sigma^\ast \setminus L$, i.e. the set of all strings over $\Sigma$ which are not in $L$.
In the particular case of
$$\mathrm{Sat} = \{\langle \psi \rangle \in \Sigma^\ast \mid \langle \psi \rangle \text{ encodes a satisfiable propositional formula } \psi \}$$
we find that
$$\overline{\mathrm{Sat}} = \{w \in \Sigma^\ast \mid w \text{ does not encode a satisfiable propositional formula } \psi \}$$
and hence, $\overline{\mathrm{Sat}}$ consists of all strings which represent unsatisfiable propositional formulas (i.e. ones that have no satisfying interpretations) and all strings which don't represent formulas at all.