means The blank border around the edge of a page, or by extension the amount of room, leeway, or difference you have to work with.

from From Latin 'margo' (genitive 'marginis'), meaning an edge, border, or brink — the same root that gives us 'marge' and is a likely cousin of 'mark' and 'march' (a frontier). It traveled through Old French before settling into English, carrying its sense of the rim of things: the edge of a page, the edge of profit, the edge of safety.