Immunity is the idea that an article cannot possibly be deleted, either because no one will dispute the fact it belongs, or it meets inclusion criteria so well, that no one will dare think to have it deleted. The belief that an article can have immunity is related to the concept of inherent notability, and which a subject is notable beyond any doubt.
In theory, some subjects are obvious for inclusion in an encyclopedia. No one would dispute the presence of an article on a country, an ocean, a head of state, a living thing, a space body, etc. that is household vocabulary. You would find these in any paper encyclopedia where inclusion is much more limited. There is no shortage of reliable sources on them available. The possibility of deleting them would never cross one's mind.
At the same time, any registered user has the right to propose any article for deletion. Of course, this should only be done in good faith, and to propose an article for deletion when one knows out of common sense that no one would ever support its deletion could be seen by some as disruptive.
Can an article be immune to deletion?
That is a big question.
If an article meets Wikipedia's inclusion guidelines very strongly, one may wonder if it could possibly be deleted.
The general notability guideline is the main criteria for determining worthiness of inclusion. If a subject by far meets this guideline, one may assume that it is immune from deletion.
But there are other reasons articles do get deleted. Many AfD debates hinge not over whether an article has the sources needed to meet the general notability guideline or not, but on some other issue.
Some examples are:
There are many things that Wikipedia is not
There is a separate guideline page called "What Wikipedia is not". This page lists all types of material that should not be included on the basis that it fails to meet the general notability guideline. But it also lists other types of articles that should not be written, and even if well-sourced, may face deletion. Some of these are:
- News: Wikipedia is not news. There are countless news events that find their way into the local, national, and international news every day that are reported by often hundreds of sources. It is easily possible to take any of these events and write a Wikipedia-style article on it, with a mountain of unique footnoted citations. You have every reason to claim it's notable. Not so. There is a guideline on What events are worthy of inclusion and which ones are not. So before you go create an article on the story that appeared in 109 newspapers, find out if it really belongs.
- Guides: So, you know all about how to change the air filter of a car. You don't need a mechanic. It is a simple repair anyone can do in their own driveway. You have plenty of sources to prove that. The local library has 5-6 books that show how to do so, and a national bookstore chain carries even more. Safe, you think. Wrong! Wikipedia is not a guide. The purpose of the encyclopedia is not to provide how-to instructions or advice. There are sister projects that can be used for this purpose. But not the encyclopedia.
- Directories: You may think of writing a Wikipedia page listing all the residents of London. All the information would be verifiable. There are public records listing them. There are phone books. There are other listings available to anyone. But you better keep it to those. Wikipedia is not a directory. Lists can be made on Wikipedia, but only when complying with this guideline. Lists that are tolerated are usually lists of other Wikipedia articles and are here either for navigational purposes, or as an extension of notable information that could be listed in a parent article if not for its length. Lists of subjects that are not notable otherwise are generally not accepted. But remember that the subject itself, phone books, may or may not be not notable.