Deep value means buying a business for less than its assets or normalized earnings justify. The danger is the value trap: a stock that's cheap because the business is genuinely deteriorating.
The separators are durability and balance-sheet strength — a low multiple on a stable, cash-generative business is opportunity; the same multiple on declining revenue and rising debt is a trap.
Margin of safety is the discipline that protects you: the price you pay, not just the quality of the business, decides your return when you're wrong about the future.