Downvoting has two aspects: reducing the visibility of a post and discouraging the poster from making similar posts in the future. This can be used maliciously (though to limited effect) against viewpoints you merely disagree with. But it does have a legitimate use as well, when harmful or unhelpfully disruptive information is posted.
A few examples to clarify these guidelines:
Spam and inappropriate material (this isn't 4chan).
Advice that is well known to be wrong and/or is very likely to be harmful if followed, such as advising someone to embed raw SQL query strings in web application parameters or to exclusively use javascript for client side input sanitation (both of which would open up anyone to huge security vulnerabilities).
Unnecessary or unfunny jokes (live by the sword, die by the sword, jokes rarely improve discussion).
A few examples to clarify these guidelines:
Spam and inappropriate material (this isn't 4chan).
Advice that is well known to be wrong and/or is very likely to be harmful if followed, such as advising someone to embed raw SQL query strings in web application parameters or to exclusively use javascript for client side input sanitation (both of which would open up anyone to huge security vulnerabilities).
Unnecessary or unfunny jokes (live by the sword, die by the sword, jokes rarely improve discussion).
Completely off-topic material.