Lets say for example you are trying to convert temperature units measured in Celcius to Kelvin
You know that 0ºC=273.15ºK
If x represents the grades in Celcius, the function f(x) will be:
f(x)=x+273.15
Where its output will be in Kelvin.
The input of the function will be the original variable and the output will be the objective variable.