Returns a string with backslashes before characters that need to be quoted in database queries etc. These characters are single quote ('), double quote ("), backslash (\) and NUL (the NULL byte).
An example use of addslashes() is when you're entering data into a database. For example, to insert the name O'reilly into a database, you will need to escape it. Most databases do this with a \ which would mean O\'reilly. This would only be to get the data into the database, the extra \ will not be inserted. Having the PHP directive magic_quotes_sybase set to on will mean ' is instead escaped with another '.
The PHP directive magic_quotes_gpc is on by default, and it essentially runs addslashes() on all GET, POST, and COOKIE data. Do not use addslashes() on strings that have already been escaped with magic_quotes_gpc as you'll then do double escaping. The function get_magic_quotes_gpc() may come in handy for checking this.