What is "to stand in the gap"

Gap \Gap\ (g[a^]p), n. [OE. gap; cf. Icel. gap an empty space, Sw. gap mouth, breach, abyss, Dan. gab mouth, opening, AS. geap expanse; as adj., wide, spacious. See Gape.]

1. An opening in anything made by breaking or parting; as, a gap in a fence; an opening for a passage or entrance; an opening which implies a breach or defect; a vacant space or time; a hiatus; a mountain pass.

   Miseries ensued by the opening of that gap.
   --Knolles.

   It would make a great gap in your own honor.
   --Shak.

2. (A["e]ronautics) The vertical distance between two superposed surfaces, esp. in a biplane.

   Gap lathe (Mach.), a turning lathe with a deep notch in the bed to admit of turning a short object of large diameter.

   To stand in the gap, to expose one's self for the protection of something; to make defense against any assailing danger; to take the place of a fallen defender or supporter.

   To stop a gap, to secure a weak point; to repair a defect.