what is the meaning of bulge

Dear student,
 Bulge  is to swell outward; to extend out into a lump or mound.
Equatorial bulge, a bulge around the equator of a planet due to rotation. 

The bulging of equator and flattening at poles is due to rotation of earth. The rotation of earth creates a centrifugal force which is pseudo effect counter balancing the inward acting centripetal force. This pseudo effect pushes the earth outward, bugling from earth, and flattening at poles. 

Regards