1337 is a meme that is often used to replace numbers with 4 digits. It is a language where you speak English but you write any eligible characters with a number. It should make you look like a hacker. It is also common in the Baldboys community to send each other happy 1337 at 13:37 which can only be done in a country using 24-hour time. Below is a list of character that can be replace by numbers in basic leet, in advanced leet you can replace every character with anything.
1 = L
2 = Z
3 = E
4 = A
5 = S
7 = T
8 = B
0 = O