means A barrier of horizontal or vertical bars, typically along stairs, balconies, or edges, meant to keep people from falling or wandering off.

from From 'rail,' which came into English through Old French 'reille' (a bar or rod), itself from Latin 'regula' — a straight stick, a rule, a ruler. So a railing is, quite literally, a thing that keeps you in line. The '-ing' is the same collective ending that turns 'rope' into 'roping' — here naming the whole assembly of rails rather than a single bar.