Hi,
Solid angle is the spatial angle. It is different from the planar angle. An example is the angle at the vertex of a cone. An antenna radiates energy in a solid angle defined by the azimuth angle and elevation angle. Mathematically, the value of solid angle can vary from 0 to 4Pi.
Imagine that a power 'P' Watts is fed to an antenna whose radiation pattern has a solid angle of Φ. Then the average radiated power per unit solid angle is P/Φ. In general, the radiated power per unit solid angle at any point in space will be a function of the direction of that point in terms of azimuth angle and the elevation angle.
Hope this helps.