@Mogima He's deleting stuff from a table that presumably has customer data, but he didn't specify a condition for what he wants to delete, so it'll just delete everything in the table. (Disclaimer: I know like nothing about MySQL)
it's nice that the author made this part "realistic" with the SQL statement and all, but i hope they also consider that every company that has something to its name does regular backups which makes this not harmless, but not as bad as "deleting everything" since they at most lost like 1 week of data
@Mogima & @Lyanhyrt: The line of code is from a database-oriented language called MySQL. It's a query to database and translates into "delete all entries from the customers database table". This is a MAJOR no-no dealing with databases, second only to the DROP instruction (a.k.a. destroy a table and everything on it).
However, a thing: you cannot simply walk into Mordor use a query, it has to be written and coded beforehand. So what he's seeing is probably not his code, it's somebody else's. Probably is that query what he thinks that doesn't belong there. (Using queries like that isn't unusual in testing phases) That again, maybe he's cheching the DB so maybe he's in a world of trouble.