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Binding refers to assigning something that was previously open to many options.
- free variables in math
- variable binding in programming, etc.
- specific meanings in linguistics
In linguistics it often refers to the binding of bound morphemes, bound roots, bound stems.
You could also see sentences as having unbound variables in things like pronouns and other forms of reference.
Referent resolution is often required for binding, and referent resolution may itself require pragmatics, common sense, and such.
Pronouns are often unbound. In Jane found his book, his probably is probably an endophoric (reference to something within the text) anaphora, probably referring to some/the recently mentioned man.
Agreement plays a role too -- consider that in Jane found her book, the book could be her own, or another woman's.
You could argue that the reflexive reference can be bound to an unbound reference. Consider They hurt each other: it is clear that the group did something among themselves, but not what that group refers to.