To lock an evolving software distribution or document against changes so it can be released with some hope of stability. Carries the strong implication that the item in question will "unfreeze" at some future date.
There are more specific constructions on this term. A "feature freeze", for example, locks out modifications intended to introduce new features but still allows bugfixes and completion of existing features; a "code freeze" connotes no more changes at all. At Sun Microsystems and elsewhere, one may also hear references to "code slush" - that is, an almost-but-not-quite frozen state.