This is literally the electronic signal rate of a piece of hardware.
I'm sure you understand that everything in your computer is ones and zeroes. When we send these ones and zeroes down a network cable, we do so by fluctuating the voltage over a pair of wires. A "low" voltage means a "0" and a "high" voltage means a "1".
When two devices participate in networking, they must agree (at a physical level) on the signalling rate to listen for voltage fluctuations.
If one end is sending 1000 cycles per second and the other end is listening expecting 100 cycles per second, then communication will not make sense. It would be likeonepersontalkingtoofasttounderstand and aaannoootthheerr tttaalllkkiinnnggg rrreeeaaalllyyy sssllooowwwllyyy and computers cannot deal with this.
A signal degrades over distance as the copper wire attenuates. This is why you cannot send ethernet traffic over a single cable further than 100 metres, and why everyone's home ADSL gets slower the further they are from their telephone exchange.
It is much easier to send longer pulses more precisely. It is much easier to slow the signalling rate down and get consistent data. You can probably do a few tens of megabit between your house and the exchange (given the right xDSL hardware on each end), but you cannot do gigabit because the phoneline suffers too much interference to transmit this much signalling reliably. You can however do up to 10 gigabit on a nice shielded CAT6 ethernet cable.
Eventually the distance required to transmit signals makes copper wire economically unfeasible so optic fibre is used. We've now changed from sending atoms along a resistive copper wire, to sending light down a glass tube, so our signalling rate is high, and our attenuation is low when compared to copper.