It doesn't determine daily visitors. It merely estimates the number of searches performed in Google for a search phrase during the previous month. Make sure that you select phrase match or exact match when using the Keyword Tool or you will be working with almost meaningless numbers.
Daily visitors are a result of a good rank position and how enticing you make the HTML title tag and snippet. Retaining those visitors is the result of a good design and useful content.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin