It's a single-piece fuse — set of 1 — designed for motor branch-circuit protection where the aM characteristic allows it to ride through motor inrush while clearing sustained overloads and short-circuits.
The 1000 A rating at 500 V AC is the continuous current the fuse can carry without nuisance opening, and the voltage it can safely interrupt under fault. The NH4 size is a standard DIN blade form factor — the fuse base and holder must match NH4. The aM curve means it's intended for motor circuits: it holds through the starting surge (typically 6–8× FLA for a few seconds) but opens on a hard short or sustained overload. Don't substitute a gG (general-purpose) curve here — the time-current characteristic is different and may nuisance-blow on motor start.