Put simply, your turntable's cartridge translates vibrations into sounds. For the cartridge to do its job properly, your turntable needs to be isolated from external sources of vibration, like speakers.
Speakers are typically the most significant source of vibration in a stereo system. If you hear feedback or distortion when you turn up the volume, then your speakers and turntable might not be sufficiently isolated from each other.
We recommend putting your speakers and turntable on entirely separate playing surfaces. Speaker stands are a great option (we're partial to these Sanus stands). If you're short on space, you can cheat a little by putting isolation pads under your speakers - this works best for small speakers being used at lower volume levels.
We cover speaker positioning in more detail here.