The leetspeak arised by the word elite which was shortened to leet and written 1337 in leetspeak. Leetspeak can be hard to read and was used as a secret code of certain groupings in former computer scenes. Today, the leetspeak is mostly used ironically.
Leetspeak arose to prevent the automatic read out of mails and messages by algorithms. Machines can not detect the word S3cr3t as Secret, but humans can (linguistic redundancy). Because of this, leetspeak has a sort of protection against spam filters working with a blacklist. Passwords using leetspeak are much safer against brute-force attacks with word lists.
Type anything into the top input field and it converts to leetspeak as you type. It works the other way too — paste leet text into the bottom field to translate it back to plain English. Tick the pro-mode checkbox for more aggressive character replacements, where letters get swapped out for multi-character symbol sequences. Voice input is supported in Chrome and Edge via the microphone button next to the input field.
A few quick examples of what the converter does: "hello world" becomes "h3110 w0r1d", "elite hacker" becomes "31173 h4x0r", and "leet speak" becomes "1337 5p34k". In pro mode the substitutions go further — letters get replaced with sequences like |_| for u or |\| for n, making the output much harder to read at a glance.
Mostly for fun these days — gaming usernames, handles, messages between people who grew up on the early internet. Some use it to get past basic word filters since automated systems often struggle to recognize leet-substituted words. It also shows up regularly in memes and retro-internet nostalgia, and the domain 1337.me itself is a small nod to that culture.