Google sandbox is a term that was coined based on how Google typically treats new websites. It's the early stages where your site is very new and google doesn't send you much traffic. You may see your rankings go up and down during this period. One day you could rank #4, the next drop to #15. During this time, it's important you just keep building content and promoting your site. Over time your rankings will stabilize.
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Don't put the cart before the horse. Plan your website, thencreate it.