Margin of safety is the gap between a stock's price and your conservative estimate of its worth. The wider the gap, the more room you have to be wrong about the future and still do fine.
It shifts the focus from forecasting growth to not overpaying. A great business bought at a rich price can be a poor investment; a fair business bought cheaply can be a good one.
Build the margin into the buy decision, not the hope — screen for low multiples on durable cash flows, then demand a discount before acting.