Preamplifiers, also known as 'preamp' or 'phono stage', take the weak signal from the turntable's cartidge, amplify and add EQ. When the record was cut, engineers squelched bits of the highs and lows from the audio spectrum. The 'RIAA EQ' folds the missing bits back in. Few turntables come with a preamp built-in, so you may need to buy one of these as well.
What's a phono preamp, and why do I need one? What is RIAA EQ, anyway? Which preamp is best for me? Your answers here.