NSF certification means a product was independently tested to a public health and sanitation standard — for foodservice, that it's designed to be cleaned, won't harbor bacteria, and uses safe materials. Health departments rely on it as shorthand for "commercial-grade and cleanable."
Most jurisdictions require food equipment to meet an applicable NSF/ANSI standard (or an equivalent third-party mark such as ETL Sanitation). That's the main reason residential equipment fails: it isn't certified to these standards. Look for the mark stamped on the unit or its data plate. Common standards include NSF/ANSI 2 (food equipment) and NSF/ANSI 7 (refrigeration).