That notification is designed to encourage people to improve their questions. In this case it was activated by the word "best".
"Best" is a vague word. Its meaning is entirely open to individual interpretation. To one person, "Best" might mean "most simple"; to another person it might mean "fastest performance", and to another "lowest memory usage". Another person might interpret it to mean "the most elegant way", which itself is still a little vague.
Thus, if you find yourself phrasing a question this way, go back and remove "best", and tell us what your real requirements are. Be specific about what you need.
Questions which rely heavily on individual interpretation of what the question is asking for are too subjective and not a good fit for the site.
Note: this notification is a recommendation, not an order. Your question will not be blocked, or automatically closed by the system. It may well be that you have expressed your needs very specifically, in which case your question probably won't be closed. In most cases, however, the word "best" is an indication that your question isn't specific enough.