"what on earth is..." (explanation)
Tony (eronie),
that is a brand name by Junkers for their continuous flow gas water heaters (most of the times in form of a combined gas central heating/ gas boiler (cont. flow). It acts on demand, so when you turn on a faucet, the flame will ignite and heating will be directed to the water flowing through. Once you are done, the unit resumes its regular central heating tasks, thermostatically controlled.
The word "geysir" (also geisir, geiser, pronounce "guy-zir" or "guyzR" is Norse/Islandic and describes those natural northern hot water fountains (through volcanic action) which allow people to jump into the lake and have a steaming hot water bath even in the coldest winter - from the Vikings' times up to today.
Junkers took it as a brand name, along with the special spelling included.