For a given interpretation of a language L, a well-formed formula of L without free variables (called closed wf or a sentence) represents a propostion that is true or false, whereas a wf with free variables may be satisfied (i.e., true) for some values in the domain and not satisfied (i.e., false) for the others.
(Introduction to Mathematical Logic - page 58)
Denumerable sequences: a denumerable sequence s = (s_1, s_2, s_3,...) is to be thought of as satisfying a wf B that has <x_j1, x_j2,..., x_jn> as free variables (where j1 < j2 < ... < jn) if the n-tuple <s_j1, s_j2,... s_jn> satisfies B in the usual sense.
(Introduction to Mathematical Logic - page 59)