It is a more heavy duty electrical supply typically used in light induatrialand agricultural feeds to run heavy motors, welders etc. The normal domestic feed is single phase - not sure about why they are called single and three phase though
I'll try not to be too technical. This referes to an AC (Alternating Current) supply where the current oscillates like a wave motion. This means that the available electricity varies dramatically as the wave goes up and down. A 3 phase supply has 3 separate "waves" which overlap each other, with peaks in different places to increase the average availability. If you want a more technical description, post a request here.
In the UK there are 3 electrical phases, red, yellow and blue. Your house sockets will be connected to one phase. Connecting to one phase + neutral = 230v, the voltage required to run most household appliances is 230v. Connect to three phases + neutral = 415v, the voltage required to run many industrial type size appliances is 415v.
Among other things, the three phases allow you to have each wire giving only 240 V, but allows you to extract the higher voltage from them. This is safer than having 415 V knocking about the place.