Ionisation

As Miss Garrett told the Second Doctor, Jamie and Victoria in the year 5000, ionisation "is a method of intensifying the Sun's heat onto the Earth's surface, but into particular areas". The Doctor simplified this explanation for Jamie, saying that it was "like a magnifying glass". It was also the method the Doctor recommended that Clent take to solve the problem of the Second Ice Age.

However, the process was not without its difficulties. It required perfect control of the ioniser technology, or areas of the Earth could become uselessly super-heated. As Clent pointed out, ionisation could "melt rocks". Moreover, it had to be done extremely carefully so as to prevent widespread flooding.

A series of ioniser bases were dotted over the Earth's surface, and these normally had to work in coordination with each other in order to effect what was called "the World Plan" — that is, the effort to carefully re-heat the Earth. However, to counteract the Ice Warrior threat, the ioniser base under Clent's control in Europe used its ioniser independently of others to defeat the Ice Warriors with intense heat. (DW: The Ice Warriors)

Behind the scenes
The "ionisation" on display in The Ice Warriors has nothing to do with real world ionisation. Ionisation is about creating an ion, a process which doesn't produce significant heat. Ionisation, as understood by real world physicists, is usually about changing the number of electrons in an atom, thereby giving it a different charge. Despite what the Doctor tells Jamie, it is not — in any way — "like a magnifying glass" focussing the sun's heat.