Someone or something may not pass or enter from one area to another.
The particular catchphrase “You shall not pass” originates from The Lord of the Rings trilogy but is used widely in memes and internet-slang.
In the meme-world we can see different variations of the phrase, e.g. Gandalf from LOTR being a teacher telling his students that they shall not pass.
In the movie The Fellowship of the Ring – the first movie of the LOTR trilogy – the main characters are fleeing from a Balrog.
To save the others, Gandalf the Grey turns back to fight the monster, when he yells the iconic phrase “You cannot pass”. The phrase later turned into the famous catchphrase “You shall not pass”.
After the famous sentence, Gandalf the Grey fights the Balrog, dies, then reincarnates as Gandalf the White, a more powerful wizard, making the catchphrase epic.
An image of the exact scene from the movie, with the catchphrase written on it, is regularly used as a meme on multiple meme-sharing platforms such as Reddit and 9GAG, but multiple other social networks as well.
It is mostly used by the younger generations aging 15-35, and obviously LOTR fans.