http://dev.mysql.com/doc/refman/5.1/en/comments.html
Comments of the form /*! stuff */ are treated as comments by other RDBMSs, but MySQL will read what’s inside the comment and execute it as SQL. You can use this to take advantage of MySQL-specific features even using code that might be run against other RDBMSs. For example you could use /*! ENGINE=INNODB */ in a CREATE TABLE query.
The numbers are optional and if you use them then MySQL will ignore them if its version number is less than the number (with dots inserted in the appropriate places).