In optics, the Numerical Aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the property that it is constant for a beam as it goes from one material to another provided there is no optical power at the interface.
But in optical fibers, Numerical Aperature (NA) is a critical performance specification. The higher the value of NA more is the light collected for propagation in the fiber. Numerical aperature is thus considered as a light gathering capacity of an optical fiber.