When a patent lapses, its owner loses the exclusive right to the invention, i.e. the right to stop others from making, using, importing into, or otherwise exploiting product according to the invention in the country where the patent was granted. If someone else starts to exploit the invention then the owner of the lapsed patent is unable to enforce the now lapsed patent right. However, take care because:
1. It’s possible to have lapsed patents reinstated in some circumstances
2. Someone else may have an earlier, unlapsed, patent that is in force and which has claims that are sufficiently broad to cover the invention; and
3. There may be other intellectual property rights associated with the invention that haven’t lapsed. For example, design registrations that protect the visual appearance of the product or copyright that protects software that the product uses.
The day you file your patent application the content of the application is officially recorded along with that day’s date, which becomes the official filing date. The application is then accorded an application (or ‘serial”) number and any earlier priority date that you claim in your application is officially noted. All of these actions are necessary for your application to be recognised in the ‘queue” at the Patent Office so that it has priority for the granting of a patent over some one else’s later filed application for the same invention. So to that extent you are protected in that a person applying after you, with a later priority date claim, won’t usurp your place in the queue.
Filing a patent application won’t give you any standing to sue anyone else for patent infringement until a patent has actually been granted. You can’t launch patent infringement proceedings unless you have a granted patent.