Full Idea
On a substitutional semantics of a first-order language, a domain of objects is not specified. Variables do not range over objects. They are place markers for substituends (..and sentences are true-for-all-names, or true-for-at-least-one-name).
Clarification
'Substituends' are things that can inserted for the place markers
Gist of Idea
Substitutional semantics has no domain of objects, but place-markers for substitutions
Source
Ruth Barcan Marcus (Nominalism and Substitutional Quantifiers [1978], p.165)
Book Reference
'Philosophy of Logic: an anthology', ed/tr. Jacquette,Dale [Blackwell 2002], p.165