Potential falsifiers are a specific class of observational statements, they’re used to test universal statements. In Popper, they’re called basic statements.
The logical role of basic statements is laid within a logical falsification: we use them to falsify universal statements, not to verify them, since to do the latter we’d need infinitely many observations.
our “basic statements” are anything but “basic” in the sense of “final”; they are “basic” only in the sense that they belong to that class of statements which are used in testing our theories.
K.Popper, Conjectures and Refutations, p.338