Its a 'hey dummy' warning. Unclick the 'warn next time' box and you will be fine.
Basically means that your computer is nt fast enough to play the footage back without adjusting resolution and frame rates. Only important if you are capturing a live stream to tape.
Use external firewire hard drive to store your video, no USB drives. Don't capture or render any video & audio to your primary Macintosh HD, it slows things down.