The modern concept of the patent was developed in the United States. It is intended as an agreement or bargain between the inventor of an idea and a national government.
The basis of the bargain is that if the inventor of a novel idea wishes, they may describe details of the idea in an application for the patent, and if the application is approved, the inventor is granted an exclusive right to the use of the idea for a limited period, perhaps 20 years. After this period, the exclusive right lapses, and other people may use the idea freely, without any agreement or payment to the inventor.
So in exchange for giving the inventor exclusive use of the invention for a certain period, the government persuades them to reveal full details of it, and when the patent expires at the end of the period, the idea is then available to anyone else ("in the public domain"), so it can benefit the community at large.
There are certain rules:
The inventor must be a 'real' person, that is, not a company or other organization, as an organization is not regarded as capable of original thought or invention in its own right. However, a patent holder can sell or license or assign use of the patent to a corporate organization.
The invention must be 'novel' or new, in the sense that it has not been previously described in any public way (published or 'disclosed'). This requirement for non-disclosure generally applies over the whole world and to all media. For example, a patent application for a method of lifting sunken ships by filling them with light flotation devices was refused because a patent examiner (responsible for considering the patent on behalf of the government) had seen a Mickey Mouse cartoon film in which a ship was raised by filling it with table-tennis balls.
There are also other considerations:
If granted, the patent only applies in the country which grants it (there is no such thing as a 'world patent'). However, if a patent is granted in one country, the patent holder generally has priority in registering the idea in other countries.
Obviously, submitting the invention to a patent examiner does not count as 'disclosing' it. Nor does describing the idea in a private letter or other confidential document sent to someone else. However, if the recipient then published the idea, its 'novelty' and patentability would be lost, so recipients are normally required to abide by a 'non-disclosure agreement'
To be patentable, the idea must usually describe a process or a device, rather than a general concept or end product. Thomas Edison held over a thousand US patents, but perhaps his greatest invention, the idea of an industrial research laboratory, could not be patented. A newly-bred fruit variety may be granted what is sometimes loosely described as a 'plant patent'. However this is not a patent at all, and has protection under quite different sorts of laws, 'Plant Variety Rights'.
Getting a patent granted can be an expensive and long-drawn-out process, taking many years. An applicant for a patent can mark their product 'patent applied for' or 'patent pending' (this form is discouraged) but they have no rights under the patent until it is granted.
The government does not undertake any action to enforce the rights of a patent holder, the patent holder must sue for damages against the offender in a court of law.
NOTE: The above is a brief and simplified explanation of the main points involved, and is not to be taken as having any legal validity or being necessarily accurate.