It is a cartridge-style fuse for protection of power circuits in industrial distribution panels.
The gG curve means it provides full-range overcurrent protection (both overload and short-circuit) for cable and general equipment, not specifically for motor starting (which would call for aM). The NH4 blade form factor fits standard DIN NH fuse holders and disconnectors common in European-style panel builds.
Where this class of fuse is used
NH gG fuses of this size are typically installed in industrial switchgear, main distribution panels, and feeder circuits where high short-circuit capacity and selective coordination with downstream devices are required. The 1250 A rating places it at the incoming supply side of a large installation.