Here are the two important rules:

• Use the "very low quality" flag only for posts that, for example, read like:

u ned put foo bar clik link blow it solv ur pr0bzelm!
regrs

Basically, anything that can not be salvaged through editing because it's not understandable, terribly poor English, or lacks formatting, and just needs to be deleted. It's not the "one flag fits all" thing, but if nobody could rescue that post, it'll have to go.

• Do not flag posts that are wrong or technically incorrect.

What to do instead if you think the answer is wrong:

• Leave a comment explaining exactly what's wrong.
• Downvote. (That's what these votes are for. Moderators shouldn't judge whether posts are technically correct or not.)

When should I flag as "very low quality"?

Here are the important rules:

• Do not use this flag as "I don't like that post".

• Use the "very low quality" flag only for posts that, for example, read like:

u ned put foo bar clik link blow it solv ur pr0bzelm!
regrs

Basically, anything that can not be salvaged through editing because it's not understandable, terribly poor English, or lacks formatting, and just needs to be deleted. It's not the "one flag fits all" thing, but if nobody could rescue that post, it'll have to go.

• Do not flag answers that are wrong or technically incorrect.

• Do not flag questions that need to be closed as "very low quality". If there's a matching close reason, choose that instead. If not, and the post is otherwise readable, let it be.

What to do if you think an answer is wrong

• Leave a comment explaining exactly what's wrong.
• Downvote. (That's what these votes are for. Moderators shouldn't judge whether posts are technically correct or not.)