That means to use the A0 pin for analog input (ADC0 on the schematic). Pin 0 used in analogRead(0) will be translated to pin A0. It really should say #define Vin A0 to be more in line with the best practice for Arduino programming.
I'm attempting to get my grandson involved with this type of activity and selected a Solar Panel battery charger project ( by BristolWatch.com )
In the future, please post a link to the project so that we do not have to search it out. The easier that you make it for helpers the more, and quicker you will get help.