What do you mean, and I'm on the internet all the time..
Although I will say, there are lots of things I still don't know about the internet...
If you have been on the internet for any amount of time, you would know what leet was, unless you did nothing but visit pianostreet, read your email, etc. Also, if there is something you don't understand, you can go to
https://www.google.com/, type it into the search box, and (Hopefully) get a link to some relevant information.
For simplicity sake, I will just say that 'leet' is 1337, and refers to a type of typing where letters are replaced by numbers. For example, "Hello there" would turn into "H3ll0 th3r3". It was a big fad in internet culture that is somewhat cliche now.