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1) larger wires from the meter are the feed to the main breaker.
2) the smaller breakers are the over current protection for the branch circuits. They have either black or red wires attached.
3) the branch circuits are the circuits to the lights, receptacles and appliances.
4) in the panel all the white wires attached to the termination bar are the neutrals for the circuits.
Hot wire feeds circuit and white wire returns to complete the circuit.