Step mash describes a mash schedule with multiple rest temperatures, held in sequence and usually rising — commonly a lower protein rest followed by a higher saccharification rest — rather than a single infusion mash's one fixed temperature for the whole rest.
When a protein rest actually helps
A protein rest (roughly 122-131°F/50-55°C) breaks down some of the larger proteins in the grain that can otherwise contribute haze or a gummy, slow-draining mash. It genuinely matters for under-modified malt, a high proportion of wheat, rye, or oats, or a grain bill without much husk material to keep the bed loose — well-modified modern base malt used mostly alone rarely needs it, which is part of why single-infusion mashing works fine for most standard homebrew recipes.
Equipment, not tradition, usually decides who uses this
A step mash needs a way to raise the whole mash's temperature accurately between rests — direct-fire mash tuns, RIMS, and HERMS systems (recirculating setups with an inline heater) can all do this without much trouble; a simple cooler-based mash tun can't add heat directly and either needs hot-water infusions calculated to hit each step or gets left on a single-infusion schedule instead. The decision to step-mash is often more about what a brewer's equipment can actually do than about a specific recipe requirement.