Numbers are often used as letters. The term “leet” could be written as “1337,” with “1” replacing the letter L, “3” posing as a backwards letter E, and “7” resembling the letter T. “0” (zero) will typically replace the letter “O.”
Makes sense that parents should know about these things though, especially if they are concerned about their kids activities on the net. If they saw people speaking like that chances are they wouldn’t have a clue what they were on about and would just forget about it……