The undisclosed algorithm through which all new questions are passed to determine if they can be posted on SE.

All new questions (and it seems, edits of old questions, too) are passed through a secret filtering algorithm,

Which prevents:

Which encourages:

This filter is based upon, or draws data from the low quality heuristic also used on Stack Overflow.

If you have problem with posting questions, this might be useful for you: What can I do when getting “It does not meet our quality standards”?