Phone lines send electrical versions of sound waves. The sound waves match exactly the sounds that reach the mouthpiece of your phone. It's a kind of information known as analog. The modem transforms the ones and zeroes into sound waves, and then a modem on the computer at the other end turns the sounds back into ones and zeroes. The different kinds of modems are described by how much information they can transform in a second.
Higher numbers are faster.
Old modems used to be 300 bps, or bits per second. That's 300 ones and zeroes per second. Over time they got faster, as new tricks were developed to transform the ones and zeroes into analog sound signals. Now a 56k modem can transform 57,344 ones and zeroes per second. Hope that answers your question.