Two-prong receptacle (ungrounded)
Old-fashioned two-prong receptacles connected to two-wire cables don't have the ground wires that protect people and electrical devices in case of a fault. Yet it is possible to retrofit a new three-prong or GFCI receptacle into the same outlet box without any rewiring, as long as the box itself is grounded.

Luckily, metal boxes attached to armored, or BX, cable—a type of wiring commonly found in old houses—generally are grounded; the cable's flexible metal jacket serves the same purpose as a dedicated ground wire.
Ungrounded electrical outlets leave your electronics exposed to damage caused by sudden electrical surges/fault. I always recommend replacement by a licensed electrician.