Mondas is Earth’s twin planet, the surface of Mondas is identical to Earth in every way.
Millions of years ago the Cybermens home planet Mondas drifted out of its original orbit and journeyed into deep space. During this time the Cybermen fitted a propulsion system to Mondas which allowed them to pilot the planet through space back to Earth.
In 1986 the Cybermen reached the Solar System and started transferring the Earths energy back to Mondas. Eventually their planet absorbed to much energy and was destroyed in the process.
Like the Daleks, the Cybermen have a complex and uncharted history. Different Cybermen factions colonised other planets, such as Planet 14 and Telos before Mondas was destroyed allowing the Cybermen race to survive.