Q: What does a folder with a question mark in the middle during start up means and it keeps blinking?

My mac started to freeze a lot lately so I cleared all the cookies and it still kept freezing and the keyboard force quit wasn't responding and manually turning it off wasn't working so I unplugged the power and during start up it took forever to load so finally when it loaded I deleted my hard drive and installed my mac or snow leopard disc it started to work and then it froze again I held the per button for seven secs and rebooted the system now on start up it keeps showing me a folder with a "?" Symbol in the middle ..Please help