What are the wires for on alternator?

There should be a heavy wire called the Alternator output wire, It carries power from the alternator to power your accessories and to maintain your battery. The smaller wires usually in a plug are either the alternator's voltage sense, Stator/ACurrent/Tachometer output, regulator/indicator light. or if its an Externaly regulated alternator they will be alternator Field, Stator/ACurrent/Tachometer. hope this helps