By default, phone browsers say "well crap, I'm, like, 320 pixels wide, and many websites were designed to be, like, 960 pixels wide, so I'm going to just scale down the content to 1/3 and show it that way."
By using the above meta tag, your website says "No no no, if your screen is tiny that's fine, just show this page like a 320 pixel wide browser and don't scale anything."