lets say a water company do deliver water to a city but as the city expands the water source are no longer able to keep up with the demand so the water company have to drill a new well,This is now an alternative source for water the company will still use the primary source for water but will need to use the secondary source in periods of high water demand.
The same is true for building materials if the primary supplier are not able to deliver what you need,You have to get it from an alternative source.
And as i mention in my comment,my primary supplier of electric power is hydro power,So this is the primary source of power for me and if i use more power than they can produce the rest must be supplied from an alternative source,Here this will be wind power or import from other sources.
In denmark the primary source for electric power is wind and the secondary sources is hydro power from norway or coal power from the continental europe.
the comment from fred is an answer to this question too