Typically used in IRC or IM to indicate that the previous typo should be replaced by the correct spelling of the word.

the thing that you don’t have when you’re phones percentage is over 20

Originates from the SED and subsequently the Perl construct for a regular expression substitution: s/REGEXP/SUBST/ will replace whatever matches REGEXP with SUBST. Commonly used in chat contexts by people with a Unix background and/or wannabe hax0rs to either correct their own previous typoes, or change the meaning of somebody else's statement.

Many people also tend to drop the final slash, either because they're lazy or because they don't really know where that expression comes from.