Full Idea
For the substitutional interpretation of quantifiers, a sentence of the form '(∃x) Fx' is true iff there is some closed term 't' of the language such that 'Ft' is true. For the objectual interpretation some object x must exist such that Fx is true.
Gist of Idea
On the substitutional interpretation, '(∃x) Fx' is true iff a closed term 't' makes Ft true
Source
Charles Parsons (A Plea for Substitutional Quantification [1971], p.156)
Book Reference
'Philosophy of Logic: an anthology', ed/tr. Jacquette,Dale [Blackwell 2002], p.156
A Reaction
How could you decide if it was true for 't' if you didn't know what object 't' referred to?