This is a production feature that locks the text on a particular page so that any revisions or deletions of text don't affect the numbering of any other page.
For example, in a locked script you can add text to Page 3 that runs past the length of the page. The new text won't go onto Page 4; instead, it will go onto a new page, Page 3A, and the sentence that started Page 4 before the new text was added is still the sentence that starts Page 4.
Likewise, if you delete text from Page 3 the program won't pull text from Page 4 up to close the gap as it would it an unlocked script. There would just be a gap left there and Page 3 will remain as it was.
NOTE: Locking your script is NOT the same thing as password-protecting it. Locking refers only to pagination.