The phono preamplifier, also referred to as the phono stage, is a crucial component to any analogue hifi system incorporating a turntable. Because the signal that a phono cartridge (found at the end of the tonearm of the turntable) is so fragile, a standard preamplifier/integrated amplifier operating at line-level isn't able to process the signal efficiently and accurately, this means that plugging a turntable directly into your amp will result in a very quiet and distorted sound.
The phono stage brings this delicate signal up to line-level, which the amplifier can then relay to your speakers without distortion. As with any element of hifi, each phono stage from each manufacturer has its own sound signature, can vary in quality and has better synergy with varying hifi components on the market today. Matching the correct phono stage with the right turntable can sometimes make or break a system.