That diagram does not make much sense by itself. It seems to be just part of the mower's wiring diagram.
The Briggs engine has a single wire that should be black, and a pair of wires that could be various colours, that connect to the mower itself. The single black wire is the kill wire: its only job is to stop the engine when it is grounded. People sometimes make the mistake of connecting this wire to +12 Volts when the engine is running: don't do it, that will destroy the ignition module.
The other pair of wires are the alternator output. They are connected together at the alternator, but one of them has a diode in it somewhere along its length - usually near the output connector that connects to the mower's wiring harness. The wire with the diode supplies DC, the one without the diode supplies AC.
If your engine has an electric starter, there will be a heavy wire from the starter motor. Most likely the mower's wiring will have a solenoid to operate the starter, but this is not a part of the engine.
