Holding down 3 keys while the iMac turns on?

dead of night said:

Hi. Once while troublesooting with an Apple tech over the phone, he asked me to turn the computer off and turn it back on again while holding down 3 keys. He said that when the computer beeps for the second time, to let go of the keys. My questions are:

1. What is this process called and what does it do?
2. What 3 keys do I hold down if I need to do this again?

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